"The 26 million Americans over 65 take 25 percent of all prescription drugs used in this country," according to a Food and Drug Administration report. "More than half take at least one medication daily, and many take six or more a day."

Here are some ways these drugs may interact with foods and affect overall nutrition, especially in the elderly.

*Alcohol can have potentially dangerous impact. It can deepen the depressant effect of tranquilizers, barbiturates, painkillers and antihistamines. It can cause drugs such as anticonvulsants and anticoagulants to be more rapidly metabolized, causing exaggerated responses, perhaps overdoses. It can raise blood sugar levels and interfere with diabetes treatments and drugs. It may also destroy the coating of time-release capsules, causing too much of the drug to be absorbed too soon. Diuretics. These so-called "water pills" are often given alone or with other drugs to treat hypertension (which, Tufts researchers discovered, many older people do not realize is another name for high blood pressure).

*Diuretics can deplete the body's potassium (needed for proper heart function) and other minerals, including calcium. Antacids. Even those containing calcium and taken as calcium supplements may cause constipation, already a problem for many older people because of reduced intestinal function.

*Laxatives. Some diminish the absorption of Vitamin D, which is needed to metabolize calcium. Some laxatives contain mercury, which can leech phosporus from bones. Mineral oil can deplete the body of the fat-soluble Vitamins A, D, E and K. Overuse of bran can also prevent the intestines from absorbing needed minerals such as iron.

Older people may also have different or exaggerated reactions to drugs. Valium-like drugs, for example, are much more slowly metabolized in the elderly, so over a period of days or weeks, enormous amounts can build up. This can cause behaviors that have sometimes been mistaken for senility, dementia or even Alzheimer's disease.