When Dr. Benjamin Han, a geriatrician and addiction medicine specialist, meets new patients at the School of Medicine at the University of California, San Diego, he talks with them about the usual health issues that older adults face: chronic conditions, functional ability, medications and how they’re working.

He asks, too, about their use of tobacco, alcohol, cannabis and other nonprescription drugs. “Patients tend to not want to disclose this, but I put it in a health context,” Dr. Han said.

He tells them, “As you get older, there are physiological changes and your brain becomes much more sensitive. Your tolerance goes down as your body changes. It can put you at risk.”

That’s how he learns that someone complaining about insomnia might be using stimulants, possibly methamphetamines, to get going in the morning. Or that a patient who has long taken an opioid for chronic pain has run into trouble with an added prescription for, say, gabapentin.

When one 90-year-old patient, a woman fit enough to take the subway to his previous hospital in New York City, began reporting dizziness and falls, it took Dr. Han a while to understand why: She washed down her prescribed pills, an increasing number as she aged, with a shot of brandy.

He has had older patients whose heart problems, liver disease and cognitive impairment were most likely exacerbated by substance use. Some have overdosed. Despite his best efforts, some have died.

Until a few years ago, even as the opioid epidemic raged, health providers and researchers paid limited attention to drug use by older adults; concerns focused on the younger, working-age victims who were hardest hit.

But as baby boomers have turned 65, the age at which they typically qualify for Medicare, substance use disorders among the older population have climbed steeply. “Cohorts have habits around drug and alcohol use that they carry through life,” said Keith Humphreys, a psychologist and addiction researcher at the Stanford University School of Medicine.

Aging boomers “still use drugs far more than their parents did, and the field wasn’t ready for that.”

Evidence of a growing problem has been stacking up. A study of opioid use disorder in people over 65 enrolled in traditional Medicare, for instance, showed a threefold increase in just five years — to 15.7 cases per 1,000 in 2018 from 4.6 cases per 1,000 in 2013.

Tse-Chuan Yang, a co-author of the study and a sociologist and demographer at the University at Albany, said the stigma of drug use may lead people to underreport it, so the true rate of the disorder may be higher still.

Fatal overdoses have also soared among seniors. From 2002 to 2021, the rate of overdose deaths quadrupled to 12 from 3 per 100,000, Dr. Humphreys and Chelsea Shover, a co-author, reported in JAMA Psychiatry in March, using data from the Centers for Disease Control and Prevention. Those deaths were both intentional, like suicides, and accidental, reflecting drug interactions and errors.

Most substance use disorders among older people involve prescribed medications, not illegal drugs. And since most Medicare beneficiaries take multiple drugs, “it’s easy to get confused,” Dr. Humphreys said. “The more complicated the regimen, the easier to make mistakes. And then you have an overdose.”

The numbers so far remain comparatively low — 6,700 drug overdose deaths in 2021 among people 65 and older — but the rate of increase is alarming.

“In 1998, that’s what people would have said about overdose deaths in general — the absolute number was small,” Dr. Humphreys said. “When you don’t respond, you end up in a sorrowful state.” More than 100,000 Americans died of drug overdoses last year.

Alcohol also plays a major role. Last year, a study of substance use disorders, based on a federal survey, analyzed which drugs older Americans were using, looking at the differences between Medicare enrollees under 65 (who may qualify because of disabilities) and those 65 and older.

Of the 2 percent of beneficiaries over 65 who reported a substance use disorder or dependence in the past year — which amounts to more than 900,000 seniors nationwide — more than 87 percent abused alcohol. (Alcohol accounted for 11,616 deaths among seniors in 2020, an 18 percent increase over the previous year.)

In addition, about 8.6 percent of disorders involved opioids, mostly prescription pain relievers; 4.3 percent involved marijuana; and 2 percent involved non-opioid prescription drugs, including tranquilizers and anti-anxiety medications. The categories overlap, because “people often use multiple substances,” said William Parish, the lead author and a health economist at RTI International, a nonprofit research institute.

Although most people with substance use problems don’t die from overdoses, the health consequences can be severe: injuries from falls and accidents, accelerated cognitive decline, cancers, heart and liver disease and kidney failure.

“It’s particularly heartbreaking to compare rates of suicidal ideation,” Dr. Parish said. Older Medicare beneficiaries with substance use disorders were more than three times as likely to report “serious psychological distress” as those without such disorders — 14 percent versus 4 percent. About 7 percent had suicidal thoughts, compared with 2 percent who didn’t report substance disorders.

Yet very few of these seniors underwent treatment in the past year — just 6 percent, compared with 17 percent of younger Medicare beneficiaries — or even made an effort to seek treatment.

“With these addictions, it takes a lot to get somebody ready to get into treatment,” Dr. Parish said, noting that almost half of the respondents over 65 said they lacked the motivation to begin.

But they also face more barriers than younger people. “We see higher rates of stigma concerns, things like worrying about what their neighbors would think,” Dr. Parish said. “We see more logistical barriers,” he said, such as finding transportation, not knowing where to go for help and being unable to afford care.

It may be “harder for older adults to try to navigate the treatment system,” Dr. Parish said.

Uneven Medicare coverage also presents obstacles. Federal parity legislation, mandating the same coverage for mental health (including addiction treatment) and physical health, guarantees equal benefits in private employer insurance, state health exchanges, Affordable Care Act marketplaces and most Medicaid plans.

But it has never included Medicare, said Deborah Steinberg, senior health policy attorney at the Legal Action Center, a nonprofit working to expand equitable coverage.

Advocates have made some inroads. Medicare covers substance use screening and, since 2020, opioid treatment programs like methadone clinics. In January, following congressional action, it will cover treatment by a broader range of health professionals and cover “intensive outpatient treatment,” which typically provides nine to 19 hours of weekly counseling and education. Expanded telehealth benefits, prompted by the pandemic, have also helped.

But more intensive treatment can be hard to access, and residential treatment isn’t covered at all. Medicare Advantage plans, with their more limited provider networks and prior authorization requirements, are even more restrictive. “We see many more complaints from Medicare Advantage beneficiaries,” Ms. Steinberg said.

“We’re actually making progress,” she added. “But people are overdosing and dying because of lack of access to treatment.” Their doctors, unaccustomed to diagnosing substance abuse in older people, may also overlook the risks.

In an age cohort whose youthful drinking and drug use have sometimes provided amusing anecdotes (a common refrain: “If you can remember the ’60s, you weren’t there”), it can be difficult for people to recognize how vulnerable they have become.

“That person may not be able to say, I’m addicted,” Dr. Humphreys said. “It’s a Rubicon people don’t want to cross.”

A joke about dropping acid at Woodstock “makes me colorful,” he added. “Crushing OxyContin and snorting it is not colorful.”

“Come in out of the sun,” the woman shouted to her 80-year-old husband. “You’re turning red!” The man reluctantly trudged toward the house. It was late afternoon — the end of a glorious summer day in Orange, Conn. But when he glanced down at his exposed arms, he could see that she was right. He was a bright pink, and soon he knew his arms and probably the back of his neck would be red and itchy. It was time to go inside.

He suspected that it gave his wife kind of a kick for him to be suddenly as sensitive to the sun as she had always been. He loved the sun and until recently thought it loved him back, turning his olive skin a deep brown that seemed to him a signal of health. But that spring he started to get red wherever the sun hit him. It wasn’t exactly a sunburn, or at least not the kind of burn his wife used to get that made her skin turn red and peel and hurt for days.

His sunburn was itchy, not painful, and lasted an hour or two, sometimes a little more. It certainly never lasted long enough for his dermatologist, Dr. Jeffrey M. Cohen, to see it. He told his doctor about the rash that spring when he went in for his annual skin exam. Cohen said he might be allergic to the sun and suggested an antihistamine and a strong sunscreen. He took the pills when he thought of it and slathered on the sunscreen some of the time, but he wasn’t sure it did much. Besides, who ever heard of being allergic to the sun?

Clearly Not a Sunburn

He made an appointment with his dermatologist just before Christmas. It was one of those warm, sunny days in December, before winter really sets in, so he decided to make sure his doctor had a chance to see the rash. He arrived early and parked in the lot. He took off his jacket and stood in the sunshine that poured weakly over the building. After about 10 minutes he could see that he was getting pink, so he headed into the office.

“I’ve got something to show you,” he told Cohen with a smile when the doctor entered the brightly lit exam room. He unbuttoned his shirt to reveal his chest. It was now bright red. The only places on his torso that looked his normal color were those covered with a double layer of cloth — the placket strip beneath the shirt buttons, the points of his collar, the double folds of fabric over his shoulders. Palest of all was the area underneath his left breast pocket where his cellphone had been.

Cohen was amazed. This was clearly not a sunburn. To Cohen, it looked like a classic presentation of what’s called a photodermatitis — an inflammatory skin reaction triggered by sunlight. Most of these unusual rashes fall into one of two classes. The first is a phototoxic reaction, often seen with certain antibiotics such as tetracycline. When someone is taking these drugs, the sun can cause an immediate and painful sunburnlike rash that, like a regular sunburn, can last for days, causing blistering and even scarring. Clearly this patient had an immediate reaction to the sun, but he insisted his rash didn’t hurt. It just itched like crazy. And it was gone within hours. His reaction was more like a photoallergic dermatitis, in which sunlight causes hives — raised red patches that are intensely itchy and last less than 24 hours. But that didn’t quite fit either; photoallergic reactions aren’t immediate. They usually take one or two days to erupt after exposure to light.

Each reaction is triggered by medications. Cohen reviewed the patient’s extensive med list. Amlodipine, an antihypertensive drug, was known to cause this kind of photosensitivity, but the patient had started this medicine recently, months after he first mentioned the rash. Hydrochlorothiazide, another of his blood-pressure medicines, could sometimes do this. The patient had taken this drug for years and been fine, but at least in theory, this unusual type of reaction could start at any point.

Cohen explained his thinking to the patient. He would need to get a biopsy to confirm a diagnosis. The pathology would help him distinguish the inflammation of hives from the more destructive phototoxic reaction, which destroys the skin cells. And it would help him rule out other possibilities such as systemic Lupus erythematosus, an autoimmune disease that is most common in middle-aged women but can occur in men and women at any age.

A couple of days later, Cohen had his answer. It was hives — medically known as urticaria. This was a photoallergic reaction. And it was probably triggered by his hydrochlorothiazide. He should ask his primary-care doctor to stop the medication, Cohen told his patient, and after a few weeks he should stop getting the rash.

Through the Window

The man returned to Cohen’s office three months later. The rash was unchanged. After a few minutes in the sun he would be itchy and pink, even in the dead of winter. Cohen went back to the patient’s med list. None of the others had been linked to this type of reaction. “Tell me about this rash again,” he said. The patient went through his story once more. Any time sun hit his skin, even if the sun was coming through the window, he would turn red. When he was driving, the warm touch of the sun on his arm would cause an aggravating itch. And by the time he reached his destination that skin would be bright red. Hearing this description, Cohen suddenly realized he had it right the first time. The patient had developed an allergy to sunshine — a condition known as solar urticaria.

Cohen explained that this was not a sunburn. Sunburns are caused by light in shorter wavelengths known as ultraviolet B or UVB. That form of light cannot penetrate glass. The fact that he could get this reddening through his window indicated that his reaction was triggered by light with a longer wavelength, known as UVA. This is the form of light that causes skin to tan and to age, the form used in tanning salons.

Solar urticaria, he explained, is a rare disorder and not well understood. When sunshine penetrates the skin, it interacts in different ways with different cells. The most familiar are those cells that, when exposed, produce a pigment known as melanin, which tans the skin and offers some protection from other effects of the sun. In those with solar urticaria, the body develops an immediate allergic reaction to one of the cellular components changed by sunlight. How or why this change occurs is still not known. The allergy can start in young adulthood and may last a lifetime. And it’s hard to treat.

Sunscreen, Cohen told him, is a must — even when indoors. He would also need to take a higher dose of the antihistamine that he was prescribed — at least double the usual recommended dose. Patients are also advised to wear protective clothing. Solar urticaria can be dangerous. Extensive exposure to sunlight can trigger severe reactions and, rarely, a potentially lethal anaphylactic event.

The patient received the diagnosis just over a year ago and has been using sunscreen with an SPF of 50 ever since. He doubled the dose of his antihistamine. And most of the time, the medication plus long pants and sleeves and a hat keep him safe. Most of the time. And when he forgets, he knows he can count on his wife to let him know that he’s starting to turn red again.

Lisa Sanders, M.D., is a contributing writer for the magazine. Her latest book is “Diagnosis: Solving the Most Baffling Medical Mysteries.” If you have a solved case to share, write her at Lisa.Sandersmdnyt@gmail.com.