3. Benzodiazepine hypnotics

Why they're prescribed: These drugs, also known as benzodiazepine tranquilizers, are typically prescribed to treat anxiety and insomnia and to relax muscles. Examples: alprazolam (Xanax), diazepam (Valium), estazolam (ProSom), flurazepam (Dalmane), lorazepam (Ativan), temazepam (Restoril) and triazolam (Halcion).

How they can cause depression: Benzodiazepines are central nervous system depressants. If not fully metabolized in the liver, they can build up in the body to toxic levels. The resulting "hangover effect" can manifest itself as depression. Older people are more likely to experience these residual effects because their livers often lack a key enzyme needed to metabolize the drugs.

Alternatives: Give yourself every chance to sleep well naturally: Go to bed and wake up at the same times every day, avoid meals within two hours of bedtime, allow 30 minutes before bedtime for a relaxing pre-sleep ritual, and so forth. Melatonin, a dietary supplement that helps control sleep and wake cycles, may also be worth trying.

4. Parkinson's drugs

Why they're prescribed: One approach to treating the symptoms of Parkinson's disease is to use drugs to adjust the levels of dopamine (a neurotransmitter) in the brain, as the motor symptoms associated with the disease result from the death of dopamine-generating cells in a region of the midbrain.

For example, levodopa, the most commonly prescribed Parkinson's medication, is converted into dopamine on entering the brain; it's typically combined with carbidopa, which helps prevent it from breaking down before it can reach the brain and take effect, in brand-name products such as Atamet, Sinemet and Stalevo.

Another approach is the use of dopamine agonists such as pramipexole (Mirapex) and ropinorole (Requip), which work by stimulating the dopamine receptors in the brain, thus mimicking the activity of dopamine.

How they can cause depression: Dopamine is one of three basic neurotransmitters that have been associated with depression (the others are serotonin and norepinephrine). Researchers believe that prolonged exposure to higher levels of dopamine may cause depression.

Alternatives: As many as 25 percent of all patients who are diagnosed with Parkinson's don't actually have the disease, so it's important to make sure that you're not among those misdiagnosed. Older people with essential tremors — involuntary trembling or quivering of the hands, limbs or other parts of the body — are often misdiagnosed as having Parkinson's. There's also drug-induced Parkinson's, which can often be reversed if the offending medication is discontinued early enough. (Older patients, for example, frequently develop parkinsonism after being prescribed antipsychotic drugs such as Haldol, Mellaril, Stelazine and Thorazine.) A systemic neurological examination is the best way to test for Parkinson's disease. And if you need to take levodopa, the dose can be reduced with the use of a COMT-inhibitor, a relatively new type of drug that blocks an enzyme in the body from metabolizing the levodopa before it reaches the brain.

Next: Hormone-altering drugs, stimulants and anticonvulsants. »