Caustic soda is sometimes known by it's chemical name sodium hydroxide, this is a strong highly poisonous and corrosive alkali. It is often recommended for cleaning blocked drains. Caustic soda is often an ingredient of oven and toilet cleaners. Don't use caustic soda to clear a kitchen sink, it will combine with with grease to form a hard soap which will block the drain.
NEVER keep this in the house, by that I mean store it. It is so potent and lethal it is just cooler to keep it on the shop shelves. If you do get it on your skin, flush it with cold running water for at least 10 minutes.
Alternative
Pour vinegar or washing soda with boiling water down a blocked drain. If that does not work take of the protective cover and have a go with a wire coat hanger.
