Pharmacology is the scientific field that studies how the body reacts to medicines and how medicines affect the body. Scientists funded by the National Institutes of Health are interested in many aspects of pharmacology, including one called pharmacokinetics, which deals with understanding the entire cycle of a medicine's life inside the body. Knowing more about each of the four main stages of pharmacokinetics, collectively referred to as ADME, aids the design of medicines that are more effective and that produce fewer side effects.

Medications That Cause Fatigue Some of the most common drugs that can make you tired are: Allergy medications (antihistamines), such as diphenhydramine, brompheniramine (Bromfed, Dimetapp), hydroxyzine (Vistaril, Atarax), and meclizine (Antivert). Some of these antihistamines are in sleeping pills, too. Antidepressants. One type of antidepressant called tricyclics can make you feel tired and sleepy. Some are more likely to do that than others, like amitriptyline (Elavil, Vanatrip), doxepin (Silenor, Sinequan), imipramine (Tofranil, Tofranil PM), and trimipramine (Surmontil). Anxiety medications. Benzodiazepines like alprazolam (Xanax), clonazepam (Klonopin), diazepam (Valium), and lorazepam (Ativan) can make you feel drowsy or weak for a few hours to several days, depending on which one you take.

Know What to Expect Side effects can happen with almost any medicine, says Jim Owen, doctor of pharmacy and vice president of practice and science affairs at the American Pharmacists Association. They're common with everything from birth control pills to cancer-fighting chemotherapy drugs. Many prescription drugs, for example, cause stomach problems like nausea, diarrhea, or constipation because they pass through your digestive system. Others -- like antidepressants, muscle relaxants, or blood pressure or diabetes meds -- may cause dizziness. Some might make you feel drowsy, depressed, or irritable. Some may cause weight gain. Some may disrupt your sleep or your ability (or desire) for sex. "I tell my patients that chronic symptoms are not acceptable," says Lisa Liu, MD, a family doctor at Gottleib Memorial Hospital in Melrose Park, IL. "I won't allow them to have ongoing pain or discomfort unless we have tried every alternative."