Summer is here and, along with it, the threat of heat waves and heat-related illness. Heat waves kill more people than any other single weather event, making them dangerous by themselves. But if you are among the millions of Americans taking certain medications, you may be facing an additional risk. Some drugs, taken when it’s hot, can provoke serious, sometimes life-threatening reactions.

These include drugs widely used for many common conditions, including blood pressure, asthma, depression and allergies, among others. When the temperature rises, they can impair the body’s ability to cope with heat.

“Everyone who takes medications, whether they are over-the-counter or prescription, needs to consider that their medication can put them at risk once summer arrives,” says Aaron Bernstein, a pediatrician and interim director of the Harvard T.H. Chan School of Public Health’s Center for Climate, Health and the Global Environment.

“Heat waves are getting worse with climate change,” Bernstein says. “We need to be mindful when medications mix with heat. Too much heat can make an otherwise safe and effective drug dangerous.”

Some medicines, such as diuretics, make our bodies lose water, which can result in dehydration when it’s hot outside. Others — such as beta blockers or ACE inhibitors — lower blood pressure, which makes fainting more likely in the heat. Selective serotonin reuptake inhibitors — commonly prescribed antidepressants like Prozac — can make us sweat more, also causing dehydration.

Still others — the OTC allergy drug Benadryl, for example, Cogentin, a medication for Parkinson’s disease, and Spiriva, an asthma treatment — can reduce sweating, the body’s natural cooling mechanism, which can lead to overheating and heat stroke. Finally, others — antipsychotics, for example — can hamper the brain’s ability to regulate body temperature.

Consumers can protect themselves by taking the same precautions as those recommended generally to prevent heat sickness. These include monitoring weather forecasts and — if you exercise outside — go out early in the morning or evening.

“If you have to be outside in the middle of the day, try to stay in shade and drink fluids,” Bernstein says, and seek medical help if you stop sweating or you begin to feel tired or sleepy, signs of heat exhaustion or heat stroke.

“For many people on medications, working outside may not be a choice,” Bernstein says. “If you know you have to be outside, make sure your doctor knows your job requires you to be outside. The dose may be adjusted based on that exposure, either in terms of how much or when you take it.”

Be especially vigilant if you start a new medication in the summertime, as the risk of heat reactions is higher and as the body is unaccustomed to a new drug and needs time to adjust, Bernstein says. And — most important — don’t consider halting any of your medications when it turns hot. “If you are prescribed a medication, it’s for good reason and don’t stop,” Bernstein says.

Normal body temperature is about 97.9 degrees Fahrenheit, although there is a range. Some people consistently measure lower, others a bit higher. But “when your body’s ability to get rid of heat fails, your body temperature can go up very quickly, often to 103 or higher,” which is dangerous, Bernstein says.

We keep our body temperatures constant by producing and losing heat, an important balance.

“When the outdoor temperature is higher than the skin temperature, the only heat loss mechanism available is evaporation, or sweating,” says Ying Zhang, senior lecturer at the University of Sydney’s School of Public Health. “Therefore, any factors that hamper evaporation, such as high humidity, reduced cardiac output, reduced air flow — or some drugs — can cause body temperature to rise, which can culminate in life-threatening heatstroke.”

Few studies have examined the dangerous interaction between drugs and heat, and experts say more are needed.

“Due to the lack of research in the field, it is impossible to estimate the scale of the problem,” Zhang says. “However, it is for sure that climate change will bring more health problems due to medications. Older people will be especially vulnerable because they usually take more than one drug.”

Bernstein agrees on the need for more research.

“We would be able to do much more in terms of adjusting and advice if we spent a little more time and money figuring this out,” he says.

Patients also should take care in storing their drugs because heat exposure could affect their efficacy, Bernstein says. Many drug labels contain storage instructions, although this won’t help those who need their drugs — asthma inhalers, for example — with them at all times.

“I’d be surprised to learn of any medication that was completely stable when exposed to extreme heat, but, at the same time, it’s not always clear how much of an effect heat has,” Bernstein says. “We don’t know nearly enough about how keeping a medication in a purse outside on a hot day may degrade the medication inside.”

As the world experiences more frequent and prolonged heat waves, it’s also important for physicians and other health professionals to become more aware of the problem, experts say.

“Medical schools don’t teach doctors about these risks, in part because the risks are new,” Bernstein says. “This is because climate change has brought unprecedented heat. People are experiencing heat unlike any in our history. In the past, we didn’t have to worry about medications and heat in the summer. But now, we do.”