Don't use them. Chemical drain cleaners are no friend to the environment or the drain pipes your plumbing system and can be very dangerous to use. I strongly recommend you use the manual cleaning techniques outlined in this tutorial instead of caustic chemical cleaners.
Chemical cleaners rarely work on stubborn clogs. After you try to use the caustic cleaner and it does not work, you are then left with a sink full of toxic chemicals that need to be bailed out by hand into a bucket and flushed down the toilet. Yuck.
