A blocked drain can be a source of unpleasant odours as well as a hygiene hazard. If the blockage is serious, you may have to call in a plumber, but there are some things you can use on your drains to unblock them yourself.
This is the first thing to pour into your drains to see if you can unblock them. It will not shift large blockages, but it can be enough to loosen up smaller blockages, particularly if they are caused by grease or food waste rather than physical objects (and it is worth pouring it in from time to time as a preventative measure). It works best if the drain is slow rather than blocked altogether.
You should make sure you do not splash and scald yourself by accident. You should also bear in mind that this is only suitable for metal or ceramic pipes. Using boiling water on PVC pipes may soften or loosen the joints or the seals. You can get around this by using hot water from the tap rather than boiling it.
Bicarb and vinegar
Bicarb and vinegar make a great natural drain cleaner from your store cupboard. Pour half a cup of bicarb into the drain; then add half a cup of vinegar. You should cover the drain when you have added the vinegar; it will fizz and froth out of the drain, and you want it to stay in the pipe. This should help to remove the blockage without causing any damage or danger to you; and if it doesn't work in one go, you can simply add some more until the blockage is clear.
The downside is that this will only really work if your blockage is caused by grease, and it may not work if the blockage is too big.
Chemical cleaners from the supermarket or hardware store, often made with caustic soda, are a last resort to try to clean a drain. They are stronger than home-made cleaners and should be effective at shifting a blockage.
They have the obvious disadvantage, however, of being highly toxic. You will need to protect your hands and face and avoid any fumes. They are also harmful to the environment and should be used as sparingly as possible.
If you are in any doubt, there is a simple way to clear any blockage — call in a professional plumber who can unclog blocked drains.