They're used to improve your mood usually. Like people take exstacy at raves so they can stay awake and enjoy the light/laser shows. People drink so they can let loose of all their stress.
Some people drink a lot of water and cranberry juice which will flush it out of the system. However, traces of the drugs can be found in the hair well after the drug is used, and water and cranberry...
It depends entirely on the drug. Advil and LSD are used in completely different ways but are both drugs. Basically... either medically or recreationally.