Some of the most commonly prescribed and over-the-counter drugs share the same little-known side effect: increasing the risk of heat-related illness.

Extreme heat is expanding across the middle of the US heading into the weekend, with nearly 38mn people facing major or extreme heat, according to National Weather Service forecasts issued recently. That includes states like Oklahoma and Michigan, which have some of the highest heart disease mortality rates in the nation. As climate change makes hot temperatures more frequent and intense around the world, the risk of heat-related illness is also rising.

Not only does having a heart condition increase vulnerability to extreme heat, but so do many of the medicines used to treat coronary issues. They can impact blood flow or the body’s ability to retain water. Other types of medications that can help with allergies and depression can also reduce the body’s ability to sweat, a key mechanism for keeping cool, or cause it to sweat too much.

Both the World Health Organisation and the US Centers for Disease Control list more than 10 broad categories of medication with possible heat impacts, which include dozens of drugs. But whether all drugs in those categories actually put people at risk in the heat is unknown.

“There’s very little real research that’s been done looking at the impacts of different medications on our ability to tolerate heat,” says Larry Kenney, a Penn State professor of physiology and kinesiology who thinks some of the lists of heat-linked medications out there are overly broad.

A relatively small number of human trials have examined the impacts of specific drugs. They have generally involved a limited number of participants who are usually young, healthy men. Some of the most vulnerable to heat-related illnesses include children and the elderly, the latter of whom also tend to take multiple medications to deal with chronic health conditions.

“It’s kind of limited value to say, ‘OK, clinically what does this mean for the older adult sitting in front of me with eight different medications, four of which are flagged under the categories that the WHO or CDC have put out’,” says Hayley Blackburn, a pharmacist and associate professor at the University of Montana Skaggs School of Pharmacy.

Medical experts recommend that people continue taking their medications and come up with a plan to stay cool.

“Ultimately, the best thing you can do to stay safe during any kind of climate hazard, including heat, is having your underlying chronic medical conditions well controlled,” says Catharina Giudice, an emergency physician and a climate change and human health fellow at Harvard University’s FXB Center. She recommends that those on medications that put them at risk during hot weather also plan to have access to air conditioning or have people check on them during a heat wave.

Here’s a rundown of some of the best-known drugs with heat impacts.

Antihistamines
Allergy sufferers are intimately familiar with this type of drug, which can help with symptoms of hay fever or adverse reactions to food. Not all antihistamines strongly impact the body’s reaction to heat, but the “big one” that Blackburn warns about is diphenhydramine, the key ingredient in Benadryl. Identified by the CDC as a drug with a heat risk, diphenhydramine is among those considered to have a strong “anticholinergic effect”, she explains, meaning it can decrease sweating, making it harder for someone taking it to cool down.

A review of the small number of experimental studies looking at how medications impact people’s heat response found evidence that other drugs with similarly high anticholinergic effects as diphenhydramine have increased a person’s internal, or core, temperature and reduced sweating, according to a 2024 study in the journal eClinicalMedicine Kenney co-authored.

Antidepressants
Multiple classes of antidepressants can affect the body’s ability to handle heat. They include tricyclic antidepressants — drugs such as amitriptyline and clomipramine — that can reduce the body’s ability to sweat.

Commonly prescribed classes of drugs like selective serotonin reuptake inhibitors and serotonin and norepinephrine reuptake inhibitors may have the opposite impact and increase sweating. That may seem like a good thing to keep the body cool, but the key to sweat’s efficacy is when it evaporates off the skin. If there’s so much sweat sticking to the body that it can’t evaporate, then it won’t provide significant relief.

Beta-blockers and calcium-channel blockers
Both types of medications are widely used to treat conditions including high blood pressure, heart failure and other cardiac issues. They do so by “blocking the heart’s ability to increase its heart rate,” says Giudice.

Moreover, they can also decrease the ability of the arteries to expand. Both these processes can impair the body’s ability to pump more blood and faster to the skin to help cool it down, she explains. The CDC specifically warns of the possible heat impacts of three kinds of beta-blockers, including atenolol, metoprolol and propranolol, as well as calcium channel blockers, such as amlodipine, felodipine and nifedipine.

Although both beta-blockers and calcium-channel blockers are lumped together in terms of their heat risks, Blackburn says there’s more evidence of impacts in the literature associated with beta-blockers.

DiureticsDiuretics are used to help treat people with heart or kidney failure whose bodies are struggling to flush out fluids. They can also be used to address non-life-threatening conditions, such as high blood pressure. Hydrochlorothiazide, chlorthalidone, furosemide and torsemide are some of the most commonly prescribed diuretics, according to Blackburn.

Sometimes called water pills, these drugs help “the body excrete more water through your urination”, says Giudice. Here’s why this matters: If you’re losing more water, you’re then at a higher risk of getting dehydrated, which is dangerous in the heat.