Starting Wednesday, millions of older Americans covered by Medicare will be able to get powerful weight-loss drugs for just $50 a month through a federal pilot program.

Until now, Medicare covered the medications only if they were prescribed for conditions like sleep apnea or heart issues. The new program will expand coverage of drugs including Wegovy, Zepbound and Foundayo to people with obesity who are enrolled in certain Medicare plans. Coverage will last through the end of next year.

Doctors said that while many patients are eager for these medications and that the health benefits of weight loss can be considerable, there are some unique factors to consider. The risk-benefit calculus for these drugs “is very different for older adults than for younger people, because the risk, the potential for harm, is greater,” said Dr. Rozalina McCoy, an associate professor of medicine at the University of Maryland School of Medicine who treats diabetes in older adults.

Bone and Muscle Loss

When people lose a substantial amount of weight in a short amount of time, they also lose some muscle. This is true for anyone taking an obesity drug, but is particularly worrying for older adults, who are at risk of frailty and falls. For some older adults, experts said, losing muscle might make it harder to use stairs or to sit or get up from a chair without assistance.

“Are we actually going to make them worse, to a point where now they are no longer able to be independent, no longer able to care for themselves?” said Kathryn Nicole Porter Starr, an associate professor of medicine, geriatrics and palliative care at Duke.

Rapid weight loss also decreases bone density. This can contribute to osteoporosis, a condition characterized by brittle bones that are prone to fractures. Postmenopausal women are already at heightened risk for osteoporosis because they lose bone mass as they lose estrogen.

Strength training can help preserve muscle and bone, Dr. McCoy said, though she acknowledged that lifting weights may be challenging for some older adults.

Diminished Appetite and Dehydration

Obesity drugs don’t just stamp out appetite; they can also affect thirst, which naturally decreases with age, making older adults more susceptible to dehydration to begin with. Some doctors worry that obesity drugs will heighten that problem. Dehydration can contribute to poor kidney function, Dr. McCoy said, and can increase the risk of fainting and falling.

Doctors are also concerned that patients will eat less protein on the drugs, since the medications are so effective at curbing hunger. Many older adults already eat less protein than recommended, and protein is an important component of maintaining muscle.

“If we put them on this medication that’s going to make them less hungry, less likely to eat, really significantly, potentially, reducing their protein intake — we’re setting them up for failure,” Dr. Starr said.

Other Medications

Most Americans over 65 take multiple prescription drugs. Weight-loss drugs can slow down the rate at which the stomach processes medications, which may make them take longer to kick in. Eating less can also indirectly influence certain medications; consuming less vitamin K, for example, can throw off the levels of blood thinners in the body, Dr. Starr said. Doctors may closely monitor the level of medications in patients’ blood.

When people lose weight, they may need less of some medications that are dosed based on weight. This includes thyroid hormone replacement drugs, which many older adults take, Dr. McCoy said. It’s important to be on the right dose, since taking too much can lead to heart problems and worsen osteoporosis. Weight loss may allow people to take lower doses of blood pressure or diabetes medications, which can benefit older adults, Dr. Starr said.

Losing Access

It’s not clear yet whether the government plans to extend the program beyond next year, or whether Medicare will cover obesity medications more broadly going forward. This means that many older adults may be unable to afford the medications once the program ends.

When people go off these drugs, they typically regain weight. When that happens, they don’t automatically regain the same amount of muscle they had before. “Some of that is actually replaced by fat,” said Dr. Tannaz Moin, an endocrinologist at the David Geffen School of Medicine at the University of California, Los Angeles. Older adults who stop taking weight-loss drugs may end up at their initial weight, but even weaker.

Doctors said they were talking to patients not just about best practices for starting the drugs and the benefits the medications might offer, but also about the challenges that would come with stopping them in 18 months.

Dr. Moin said that ideally there would be some reassurance “that this isn’t just going to be for a couple of months, and then you’re going to be left stranded with either the high cost or no access to these medications.”

She added: “We’re trying to keep people from falling off a cliff.”