Vertigo, a sensation of dysequilibrium or "room spinning," is often caused by inner ear problems. Conflicting messages between entering your brain from the eyes and the position-sensing organs within the ear cause these difficult-to-explain symptoms. Medications that target parts of the brain responsible for interpreting these signals can often offer relief of these symptoms as well as other associated sensations of nausea or sweating.
Histamine blockers, such as meclizine (Antivert) and diphenhydramine (Benadryl), work on receptors in the part of the brain responsible for interpreting the body's position in space. These drugs work to decrease the strength of signals being sent to the brain that cause dizziness and nausea. The most common side effect with these medications is drowsiness. Thus, you should not drive or operate machinery after taking this medication until you know how it affects you.
Phenergen and drugs like it are typically used to treat intractable nausea and vomiting by decreasing the strength of nerve input to the position-sensing and vomiting centers of the brain. Drugs in this class tend to be sedating and should not be used prior to driving or operating machinery until you know how this medication affects you.
Benzodiazepines, such as Valium and Klonopin, also work to counteract confusing signals to the brain. These work on a separate receptor in the brain that reduces signal conduction to the vestibular system within the brain. This class of drugs is also commonly used in anxiety disorders and can be sedating. Thus, you should not drive or operate machinery after taking this medication until you know how it affects you.