Mechanisms for antiemetics vary, and are listed next to their names.
Bonine/Dramamine (Meclizine hydrachloride/Dimenhydrinate): good for motion-sickness-related nausea. Side effects include drowsiness. Not recommended for children under 12, pregnant women, people with lung diseases, glaucoma, or difficulty urinating due to enlarged prostate. Mechanism: This is an antihistamine, so it suppresses acetylcholine activity in the part of the brain that controls motion and balance.
Emetrol: A sugar and phosphoric acid solution that soothes the stomach from bacterial- or viral-caused nausea. Also useful for discomfort after overeating. Diabetics should exercise caution because of the concentrated sugar. Mechanism is unknown.
Pepto-bismol (Bismuth sub-salicylate): no known side effects. Pregnant women should consult their doctor about use, because of its chemical similarity to aspirin. People allergic to aspirin will also be allergic to pepto-bismol. Mechanism: not well documented, but it is thought to be a combination of protecting the stomach lining with a coating and killing of bacteria with heavy metals and salicylic acid.