Medication can improve the quality of life for people with Alzheimer's and their caregivers. It may even delay placement in a nursing home.

Unfortunately, Alzheimer's medications don't work for everyone. Some of the most commonly used drugs work in less than half the people tested. For those who are helped, the drugs' effects often are modest and temporary. Scientists continue to search for more effective drugs or perhaps even a vaccine that might someday prevent Alzheimer's.

Of the two main types of drugs currently approved by the Food and Drug Administration (FDA) for use in Alzheimer's, one variety seems to work best in the earlier stages of the disease while the other is reserved for treatment of the later stages.

Alzheimer's disease changes the brain in many ways. One of the changes results in a decrease in the levels of acetylcholine, a chemical messenger that is believed to be important for memory, thought and judgment.

Cholinesterase (ko-lin-ES-tur-ase) inhibitors are a type of drug that improves the effectiveness of acetylcholine either by increasing the amount of it in the brain or by strengthening the way nerve cells respond to it. These drugs appear to work best in the early to middle stages of Alzheimer's.

The top three cholinesterase inhibitors seem to have similar rates of effectiveness and similar side effects — nausea, vomiting or diarrhea.

• Donepezil (Aricept). Perhaps the most popular cholinesterase inhibitor, donepezil offers a convenient once-a-day dosing not available in similar medications. This drug also appears to temporarily postpone the development of Alzheimer's in people with mild cognitive impairment (MCI), a separate memory-related condition that may precede Alzheimer's. In a recent study, magnetic resonance images show less brain shrinkage among people with MCI treated with donepezil than among people with MCI treated with a placebo. Donepezil had this effect only in people who carry a gene associated with higher risk of Alzheimer's, however.
• Galantamine (Razadyne). Approved by the FDA in 2001, galantamine is the newest cholinesterase inhibitor on the market.
• Rivastigmine (Exelon). Rivastigmine may cause more severe gastrointestinal problems than other cholinesterase inhibitors do. Doctors typically prescribe a low dose of the drug initially and slowly increase the dosage as the body becomes accustomed to it.
• Tacrine (Cognex). Tacrine has been on the market since 1993. However, it's rarely prescribed because of serious side effects, including possible liver damage.

Memantine (Namenda) is the first drug to be approved by the FDA for treatment of moderate to severe Alzheimer's disease. It works by regulating the activity of glutamate, a messenger chemical involved in learning and memory. Its most common side effect is dizziness.

Until researchers find a cure — or can halt the progression of Alzheimer's — slowing the downward spiral remains the only benefit drugs can offer. But even temporary improvements in memory or thinking ability can help prolong the amount of quality time a person with Alzheimer's has with his or her family.