How to Unblock a Sink Using Sodium Hydroxide

Sodium Hydroxide is perfect for unblocking sinks and plug holes. But why do people use it?

Sodium Hydroxide (also referred to as Caustic Soda) has fantastic degreasing qualities – meaning it can break down grease and fat, which are the most common causes of blocked drains.

You can buy Sodium Hydroxide (Caustic Soda) on Amazon where you will find a large range of sodium hydroxide products in various pack sizes and strengths.

To unblock a sink using Sodium Hydroxide, we recommend using a solution of around 10%.  To create a 10% solution of Sodium Hydroxide, simply dissolve 100g of Sodium Hydroxide in 1L of water.  If you require more or less, just use the same ratio of 1 part Sodium Hydroxide to 10 parts water.  If you wish to speed up the dissolving process you can stir the solution using a plastic stick.

We strongly recommend you use cold water to begin with.  When Sodium Hydroxide is dissolved in water it will heat up.  In chemistry terms this is known as an Exothermic Reaction.  There is no need to use boiling or even hot water to begin with because the solution will generate heat of its’ own.

Simply pour the 10% solution down the drain and let it get to work.  Leave it for a few minutes so it has time to break down the deposits in the sink.  When about 5 minutes has elapsed, slowly run some water into the sink, increasing the water pressure if the sink appears to be clear.  If it isn’t clear, leave it for a few more minutes and try again.  If the sink still isn’t clear, repeat the process of creating a solution and pouring it down the sink.  You may wish to increase the strength of the solution, however please be mindful that the level of potential hazard will rise in line with the strength of the solution – if a 20% solution will not break down the deposits in the sink you may require something other than Sodium Hydroxide (such as Sulphuric Acid, which is better at breaking down things like hair build up, although please do not pour Sulphuric Acid down the sink immediately after using Sodium Hydroxide – it will have a very aggressive reaction, and may even boil as soon as they come into contact with one another!).

Please don’t put pellets directly down the drain as they can turn into a big lump and go hard – this can block a sink even further than it already was to begin with!

It is strongly advisable to use suitable Personal Protective Equipment (more commonly referred to as PPE) when handing Sodium Hydroxide.  As a minimum we suggest using safety gloves and glasses.  It is also advisable to consult the Material Safety Data Sheet for Sodium Hydroxide before you handle the substance.

Once finished, ensure you rinse the sink thoroughly to ensure that any remaining Sodium Hydroxide deposits are removed.  You can also use Sodium Hydroxide to unblock plug holes, outdoor drains and toilets.

ReAgent supply an enormous range of Sodium Hydroxide.  Our General Use Sodium Hydroxide is ideal for unblocking sinks, along with many other applications.  We also provide numerous high-purity grades of Sodium Hydroxide solid for specialist applications such as testing and analysis, although it wouldn’t be logical to use these for unblocking drains – they are more expensive and there is no benefit in using them if you are only going to unblock a sink.

Due to the hazardous nature of this product we must use specialist couriers to supply the product, but keep in mind that using pure chemicals is much more effective than what you can buy in a supermarket.  We also provide a wide range of ready-mixed solutions, including a 10% Sodium Hydroxide Solution, although it is better value to create your own.

Rich Hudson
I'm Managing Director of a leading UK chemical supplier. My company supplies most of the chemicals I blog about, you can buy them online at I also keep a personal blog at
Rich Hudson
Rich Hudson
Rich Hudson

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  1. Stuart Boyd says:

    Hi, i need to unblock human hair from inside my shower plughole, but the pipes are made of plastic. Is sodium hydroxide safe to use in plastic pipework?

    • The Chemical Blog says:

      Hi Stuart,

      Thank you for your question – the first one on our new blog!

      It’s fine to use Sodium Hydroxide with Plastic pipes. It is also acceptable to use Sodium Hydroxide with Steel and Copper, however it is inadvisable to use the product on Aluminium as it may have an etching effect.


  2. Hello
    Just wanted to say thanks for this info.
    Have just used it in my plug holes at home and it worked a treat

    Thank you

  3. Help – I very stupidly put caustic soda into the bathroom sink and then added water (I didn’t read instructions properly) as you will know it has solidified and is now blocked. Is there a easy way out of this without calling in a plumber? The sink has a pedestal in front and the u bend not easy to reach. HELP

    • Hi Maria,

      Sorry to hear about your blocked sink! There are a couple of things you could try to unblock it but please do wear safety gloves and glasses and be very careful as sodium hydroxide is highly corrosive, especially to the eyes and skin.

      It may be possible to clear the solidified sodium hydroxide by removing any water from the sink then adding more hot water to the sink. This would need to be repeated many times but may work eventually.

      Alternatively you could dismantle the pipe work under the sink very carefully wearing safety gloves and glasses and soak the blocked parts in a bucket of warm water until the sodium hydroxide dissolved.

      Please remember though that all of the liquid will be highly corrosive especially to the eyes and skin and great care must be taken to avoid exposure.

      Hope this helps!

  4. My toilet drain is overflowing into the back garden, so lots of tissues and gunk. Can I use caustic soda for it? If so, do I pour it down the toilet or directly into the drain? Many thanks in advance.

    • Hi Joyce,

      That sounds like a major blockage, it’s highly unlikely that Caustic Soda will be able to shift it.

      I would recommend you call a drain specialist to look at the problem for you. If you’re based in the UK Dyno are a good company to call:

      Hope this helps!

  5. david rose says:

    is caustic soda septic tank friendly.thanks

    • Richard Hudson says:

      Hi David,

      I would say not. Adding Caustic Soda to your septic tank is likely to raise the pH levels unacceptably.

      Hope this helps!

  6. Hi Richard, Can i use caustic soda to clean my patio slabs? Also because the edge of the patio is raised,if when rinsing off would i need to protect the surrounding soil and plants

    • Hi John,

      Thanks for your comment. Caustic Soda will not clean your patio slabs. For this job you would usually use a brick acid or brick cleaner which you can purchase from ReAgent here:

      You will need to take care to protect the surrounding soil and plants because Brick Acid/Brick Cleaner will contaminate and harm them. You’ll also need to take great care to protect yourself when using this chemical as it’s corrosive. Make sure you use appropriate Personal Protective Equipment (PPE) such as safety glasses, overalls and gloves as outlined on the safety data sheet.

      Hope this helps!

  7. hello Richard,i have a very stubborn kitchen sink blockage,have used several less aggressive cleaners with no effect,the blockage is beyond the ‘u’ bend,have tried to pour sulphuric acid down the pipe beyond the ‘u’ but cant get the angle – thinking of ‘squirting’ the acid in with a syringe what do you think?

    • Hi David,

      Thanks for your question. I would strongly recommend that you DO NOT squirt sulphuric acid down your sink.

      I would recommend that you rinse away the chemicals you’ve already poured down the sink thoroughly with water and call a plumber to dismantle the pipework and remove the blockage. It is also vitally important that you tell the plumber about your attempts to unblock the sink with sulphuric acid so they can take appropriate precautions.

      One final point I would make is please do not pour any other chemicals down the sink until the blockage has been cleared by a plumber – this could cause a potentially violent reaction with the sulphuric acid you’ve already poured down there and cause serious damage to your sink and more importantly to you!

      I hope this helps!

  8. Hi there. I wonder if you could help. Two days ago I got a bit carried away with the caustic soda and poured it (roughly 450g) down the sink. I didn’t mix the solution with water first, just went straight ahead and added the water afterwards. I now realise that was a very silly thing to do. After reading various comments I tried removing the water then adding a load of vinegar which so far has done absolutely nothing. Any suggestions as to what next? I want to try all I can prior to ringing our letting agent. Any help would be massively associated
    Best regards.

    • Hi Lea,

      It’s likely that the Caustic Soda has solidified in your pipe. It will have also generated a considerable amount of heat when it came into contact with the water which could have caused damage to your pipe work if it is plastic. You have the right idea with adding Vinegar – this would in theory help to neutralise the Caustic Soda but you would need a huge amount for this to be effective so I wouldn’t recommend pursuing this any further.

      Due to the amount of Caustic you’ve put down your sink, the best advice I can give you is to contact a plumber and have them remove the blockage for you. It’s of the utmost importance though that you tell the plumber exactly what you’ve put down the sink before they carry out any work. Caustic Soda is highly corrosive – especially to eyes and skin so the plumber will need to take appropriate precautions.

      Good luck!

  9. Richard Power says:

    I have installed a stainless steel heat exchanger to my wood burner to produce hot water. Eventually, the heat exchanger gets clogged up with creosote and soot. It is very easy to remove the heat exchanger for cleaning, but it is difficult to abrasively clean the heat exchanger surfaces effectively. Would immersing it in a sodium hydroxide solution remove the creosote?

    Many thanks,

    Richard Power

    • Hi Richard,

      I have heard of Sodium Hydroxide being used in the past to de-coke exhaust systems on 2-stroke motorcycles so it may well work on the heat-exchanger. If you’re going to do this though it will be very important to clean off all residues before refitting the exchanger.

      Good luck!

  10. Catherine says:

    Hi Richard,

    I was just wondering if it safe to use caustic soda on ceramic sinks? I read somewhere that you shouldn’t use strong chemical such as caustic, but then read also elsewhere that it is fine.



    • Hi Catherine,

      Thank you for your comment. I would say it’s best to avoid using caustic soda on ceramic sinks – it could be unsafe so I would recommend using cleaning products you can buy in stores.


  11. Proactive Girl says:

    Great blog! I would alwyas recommend using manual ways to unclog sinks and drains prior to using chemical agents. Additionally, trying to stop junk and gunk from going down the drain in the first place is a good idea.

    For sinks and showers, you can get a strainer that collects hair and other bits. Sure, you have to clean it out now and again, but your pipes and that of the municipal sewage system will thank you.

    For kitchen sinks, using the strainer on your sink stopper will stop a lot of materials from going down the sink. Plus, garbarators, though convenient, are not a great option. Eventually this material has to be removed from the system, and better to do it up front, than spend a lot more time, energy, etc. to have the waste water treatment plant remove this material later.

    Certainly, if you have to unblock your sink, caustic soda can work, and is much more environmentally friendly than other unblocking chemicals. Just wanted to note that a little bit of preventative action can go a long way to preventing clogs in the first place.

    Cheers again and thanks for the great blog!

  12. sam clo says:

    Hey guys I live on an orchard and the house was previously on channel water my bathroom sink is clogged with mud I have pulled it appart and manually cleaned what I can but still no joy is there something I can use to break up the mud further into the pipe? Cheers Sam.

    • Hi Sam,
      Have you followed our guidelines for using Caustic Soda on the pipe? This should be able to break down some of the mud.
      If this doesn’t work, you’ll likely have a major blockage and will need to contact a plumber. Make sure you tell the plumber exactly which products you’ve already tried so they know what precautions they need to take.

      Hope this helps and good luck!

      • sam clo says:

        Yeah went through that but 2 no avail have since pulled the pipe apart again to make sure I had no caustic left just sitting there. Looks like I shall give the local bloke a call cheers for your assistance :-)

  13. Mohamed says:

    Dear Sir,
    Thank you for your advice and information.
    I live in South Africa(Johannesburg).
    Do you have a nontoxic Drain Cleaner,if you do please share this with me.
    Beast Wishes

    • Hi Mohamed,

      Thank you for your comment. The Chemical Blog doesn’t provide chemicals. However, if you visit they have a huge array of chemicals you can buy.
      Just follow the advice from our blog to create the solution.
      Bear in mind that don’t supply to residential addresses, it’s for businesses only.

      Hope this helps and good luck.

  14. Hi Richard,

    My washing machine has been smelling quite bad recently and this morning when I put it on to rinse a foul smell started to come out of my sink. Kind of an eggy, rotten smell. The sink drains quite well so there is no evidence of blockage apart from this smell and the fact that the sink also made loud gurgling noises while the machine was on. Do you think that putting caustic soda down the kitchen sink would help this, and would it cause any damage to my washing machine?


    • Hi Hannah,
      Thanks for your comment and sorry to hear about the problem you’re having!
      The smell is likely caused by sulphur deposits, which could be present in small amounts which is why there’s no problem with the drainage. However, these deposits will only build up over time and could cause more problems.
      Caustic soda should effectively clear out the drain.
      With regards to your washing machine, I can’t say whether it will damage it or not. Plus, there are some safety concerns when handling caustic soda. You could email ReAgent’s sales team on and they should be able to give you more details on caustic soda.

  15. Hi Richard, you answered Hannah’s question about her washing machine and an eggy smell which I can smell but don’t know where it’s coming from and my sink seems to be ok, sometimes draining slowly though. You mentioned sulphates but didn’t say what they are and where they are or whether one puts caustic soda down the sink to clear the ‘bad eggy smells’. Help please.

    • Hi Sheila
      I mentioned sulphur deposits which can build up in the drain. These are likely to form from a build-up of food and other materials often put down the drain, and can lead to the “eggy” smell.
      Caustic soda often clears up these deposits, along with the eggy smell.
      You can buy caustic soda from ReAgent
      Thank you for your comment.

  16. SelfHelper says:

    Hi, this advice is all great.

    I use Bicarbonate of Soda (Baking Soda) washed down with household vinegar once every few months, as a preventative measure on all the drains.

    I have used Caustic Soda mixed with water for tough blockages. I mixed it within an old (and large) Nescafe Gold Blend bottle (then dispose the bottle). However the glass gets very hot, very quickly. Obviously the glass is reacting to the mixture. Having read a lot more since then, it’s clear that a glass container is to be avoided!
    So what container should we use? Knowing the name of the material for the material would help everyone to search and buy.

    Also, most sites advise never to mix it into cold or hot, always warm water. What’s your comment on that? (your article advises cold)

    Lastly, some sites suggest stirring the water whilst adding the crystals, to keep the mixture calm. However they don’t say what the material of the stirrer should be (presumably same as the container!). Any comments on this, and suggestions as to the mixer?

    Many thanks

    • Thanks for your comment,
      Caustic soda is otherwise known as sodium hydroxide. When sodium hydroxide mixes with water it produces an exothermic reaction (produces heat) which will get very hot at the water and chemical interface. To avoid too much heat being generated add small amounts of the caustic soda into the water whilst mixing, if the solution becomes hot then allow it to stand in cold water to cool off.
      Add the caustic soda cautiously wearing eye protection and gloves as it can produce severe burns. Glass should be avoided because not all types of glass can withstand heat, therefore may break which is very dangerous to the user. Tough plastic bottles can be used providing that the solution does not get too hot whilst mixing and the plastic is compatible with the mixture. It is best to avoid mixing in metal containers especially those that contain aluminium. Use a plastic rod for stirring or a glass one that can withstand heat.

    • Angie Davidson says:

      As a soapmaker, I work with lye (sodium hydroxide) on a regular basis. I use Mason jars to mic; glass is fine but be aware that it will etch over time so you need to use something that can withstand high heat. You want to use COLD water, and ALWAYS pour your lye into your water, never the other way around. (Doing so could cause a “volcano” effect, which is quite toxic and dangerous.) Think “gently falling snow” to remember which way to add the lye.

      I know that lye should never be used with aluminum, but what about cast iron?

      • Hi Angie,
        Thanks for the very interesting information.
        Sodium hydroxide is often used to clean cast iron. Just make sure you follow the safety precautions on the packet- safety gloves etc. and you will be fine using lye with cast iron.
        The Chemical Blog

  17. SelfHelper says:

    Hi, thanks for that advice.
    Which plastic is compatible with the mixture? That’s the question.

  18. Heather says:

    Hi Richard, I have tried so far unsuccessfully to clear my shower drain of suspected blockage with Boots caustic soda solution. I think I may have to call in a plumber. There were dregs of the mixture left in the plastic bucket which have now solidified, how can I safely dispose of this and use the bucket again?
    Kind regards, Heather.

    • Hi Heather,
      sorry you had to go to the measures of calling a plumber! Regulations for disposing of caustic soda vary between areas, so you’d need to contact your local government authority to find out how to dispose of yours.
      Thanks for your question and best of luck with your problem.

  19. bassett says:

    Hi, I used Muriatic acid and then sulphuric acid in a blocked toilet but no luck. Time for the big guns.
    I then used 10% solution on a blocked toilet. Nothing! I then uped it to 20%…..Nothing. I then made up a solution of 1KG in 2 litres of hot water in a 20 litre bucket (50%).
    I slowly, bit by bit pour that in the toilet over a couple of minutes and left it for about 2 hours. I flushed the toilet and thankfully the water went with a big “GLURP”. I made another mix and have left that in the toilet just to make sure but it all looks ok.
    If you need to do this to a severely blocked toilet DO NOT POUR HOT WATER INTO THE TOILET ALL AT ONCE. It will crack the ceramic.
    Small amounts over a long time to give the ceramic time to expand a bit with the temperature change is ok but be really careful.

    • Dear Bassett,

      May I know what solution you are talking about? Is it Sulphuric acid or Sodium Hydroxide? Doesn’t the Sodium Hydroxide corrosive to your ceramic toilet bowl? I am having the same problem as yours with my toilet at home. I’ve been trying every method but not worked. Please help!

  20. Hi Richard,
    Please help! The drain outside my kitchen is blocked. I can’t get the grid off but when I poke around between the slats with a spatula, it fills as if it’s full of mud, and I can pull out a few lumps of hair. I made a solution of caustic soda 100g to 1ltr of water and carefully poured it down, then realised that there is a hole in the concrete right next to the drain, and the excess water is draining into there. Where can the hole be leading – do you think it will lead into the drain, or somewhere else? I’m worried about where the caustic soda solution has gone. I kept digging in the drain and little white lumps started forming – maybe caustic soda lumps or something else? The drain was still blocked so I put another 100g diluted caustic soda down the toilet. Nothing has made any difference and I’m really worried now that I’ve poured caustic soda under the house or somewhere else it shouldn’t go. Thanks for any advice. Claire

    • Hi Claire,
      Sorry to hear you’ve been having problems. You should contact a drainage company and get them to have a look at your situation. Make sure you tell them exactly what you’ve used so far so they know how to approach the problem.
      Good luck.

  21. Is it true that you can make a blocked drain worse with caustic soda. I understand that it will create saponin (a type of soap) when it reacts with fats. The saponin is a gel like substance that will slowly harden and completely block the drain.

    • Hi Gary,
      It depends on the method you use. We advise that you apply the mixture slowly. Don’t use hot water as it heats naturally and this can cause solidification.
      If your drain is chronically blocked, then call a plumber and let them know which products you have poured down the drain.

  22. todd misko says:

    when I have poured naoh into drains it seems to always produce an obnoxious smell, kind of reminds me of ammonia. I work at a 120 apartments building, Does the free hydrogen produce something else? Tenants have complained about the smell, which does go away after an hour.naoh is solid and I assume one hundred percent na0h.

    • Hi Todd,
      It’s likely that the smell is caused by the solution reacting with deposits inside the drain, and can’t be avoided.
      Hopefully tenants will understand that the brief smell is necessary to clean out the drains!
      Thanks for your comment.

  23. I have a problem wit my drainage i have a fish farm and my drainage where i send my bad water to now can not take much water and i have used caustic soda and soda hatch and still the problem Persist. And i also need a chemical to also stop my fish water from smelling as people around the neighbourhood are complaining

    • Hi Andrew
      You should call in an expert to try to unblock your drain. If you continue to pour different chemicals down there, you might make things worse. Ensure you let the expert know exactly what you have tried so far.
      With regards to the smell, you should attach a mechanical filter or a biological (chemical) filter if you haven’t already done so. You can buy these online. Just make sure you do your research and be certain that nothing you apply to the water could harm your fish.

  24. Martin Biruri says:

    Thanks alot

  25. Hi, I just re-rented a flat I used to live in some years ago. That means I didn’t visit it first.

    When I opened the door, the smell in the place was incredible. It was coming from the bathroom. I opened all the windows, etc, and it started to disappear. I put bleach down the plugholes, toilet, bidet, bath plug and this helped a little bit. It helps for a while, with the windows open, but it’s back to smelly when the room is closed up…

    Hmmm, in the mornings I don’t smell it so much, even though the room has been closed up all night. But when I come home from being out I really smell it.

    Yesterday, I noticed the sulphuric smell. I didn’t notice that before.

    I had a bad headache last week.

    The landlords really don’t want to acknowledge the problem or do anything. I think they probably knew about it already. I live in Spain so…things don’t work the same and the idea of calling out a plumber seemed to worry them. They left saying they would contact their home insurance people.

    My language skills are quite limited too.

    What can I do before moving again? Can I use sodium hydroxide in the bathroom. I’m on the top floor of an apartment block.

    Thank you :)

    • Hi Katie,
      Sorry to hear about your situation!
      You can try sodium hydroxide. Just be sure to read the label carefully – you may need to add it in small quantities at a time, in case there is a reaction with the bleach.
      As you live in an apartment block, there’s a chance that other tenants are experiencing the same problem. It is up to your landlord to contact a plumber as you have already tried to fix the problem yourself with bleach (and sodium hydroxide).
      Good luck and I hope your problem is resolved.

  26. Hi, I noticed that drainage hole in the door seal of my Bosch washing machine is blocked. I cannot clear the drainage by just pushing through with a wire as the drainage seem twisted inside the door seal. Can I use sodium hydroxide to clear the blockage?

    Thank you

    • Hi Kris
      Sorry to hear about your problem. Sodium hydroxide is most effectively used for its de-greasing qualities. As your problem seems to be based around the actual washing machine, rather than what is stuck inside, it may not be the solution.
      You should contact a specialist to see if they can fix it for you. Good luck.

  27. Hi Richard
    6 months ago my kitchen sink was draining very slowly so I poured down a litre of a proprietary mixture as follows: ” < 5% Anionic surfactant, non-ionic surfactant, Sodium Hydroxide; 5-15% Sodium Hypochlorite''
    This did not clear it so , after putting some water down and letting it pass through the system, used a plunger which only made water come out of the washing machine stand pipe (and sink overflow). Due to holiday it was 2 weeks before a plumber rid the blockage (using a snake ) which transpired to be about 9 feet along from the sink in the bathroom. It transpired the blockage was caused by hair rather than grease.
    Since that time however there have been 2 problems. First there is a slight smell in the kitchen- not a sewer smell or a normal 'blocked' smell (no smell in the bathroom and there was no smell before blockage). Secondly I have had eye irritation; Ive had this checked a the eye hospital who say my eyes are somewhat dry but otherwise ok (they checked the pH of my eyes). So this may be coincident or not.
    I would be grateful for any ideas as to what might be causing the smell/possibly eye irritation? Could the chemical still be around perhaps attached to some muck or the sides of the pipes? Could the pipes be damaged? I wondered, as it was/is a hair problem, about using spirit of salts?
    The water is flowing freely.


    • Hi Barry,
      I’m afraid we’re struggling to think why there would still be a smell in your kitchen or why your eyes are irritated. As you said, this could just be a coincidence, but if it gets worse then go get them checked out again.
      The best I can suggest is that you call a plumber again and explain the smell to them so they can check on your pipes. The good thing is that you know exactly what you’ve put down there, so let the specialist know this before they start dealing with it.
      If you find out what the problem was, please do let us know!
      Good luck,
      The Chemical Blog

  28. Will a 10% solution of NaOH unclog a pickle in my toilet?

    • Hi Bridget,
      It’s worth a try! Just make sure you follow all safety advice – wear gloves and try to cover your nose and mouth to avoid inhaling the solution. It’s best to carry this out at the end of the day so the product can get to work overnight when the toilet isn’t being used.
      Best of luck,
      The Chemical Blog

  29. Hi, I used sodium hydroxide to open my drain and it splashed everywhere and got on my appliances and now won’t come off. Is there anything I can do to get it off?

  30. Hi,

    We have a modern sewerage treatment plant – a Solido unit (we’re rural so no mains drainage).
    I suspect we can’t use caustic soda because of the ph levels, is there an alternative?



    • Hi Jay,

      You’d either use caustic soda or sulfuric acid. But either would have an extreme Ph, best to check with a local authority for discharge rules.
      Beware, both of these materials will get hot when mixed with water.


  31. Have used caustic soda to try and unblock a bath drain but still very slow draining what else can I try

  32. Does sodium hydroxide have a COD?(Chemical Oxygen Demand)ans if so do you what it is?

  33. help! accidently poured down the sink a mix of pure acetone and melted plastic i thought nothing of it and tried to wash it down with cold water and its blocked it.i managed to scrap alot of it out but water still wont run through. how can i get it to clear without paying out for a plumber?


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