Some of the most prescribed medications, including those used to treat depression, ADHD and high blood pressure, can increase the risk of dehydration and other heat-related conditions.

This year, at the height of Europe’s June heatwave, Ileana Okumus couldn’t understand why she was struggling so much more than other people.

“As soon as I left my apartment, my face started to sweat,” she says. Living in a top-floor Brussels apartment without air conditioning, the 28-year-old sought relief wherever she could find it – the office, local cafes. But it wasn’t enough.

“I had to wait half an hour, an hour, before I cooled down,” she says. “There was one particular day when the temperature inside the apartment reached 34C [93F]. I think I slept around 18 hours.” She found it difficult to wake up, and worried that her blood pressure might have dropped.

The intensity of the heatwave was nothing new for Okumus, who has lived through 40C [104F] summers elsewhere in the world. Yet it was the first time the heat had hit her this hard. Only after seeing social media posts from people with similar symptoms did she make the connection between the heat and the medicines she’d started taking two years ago for depression and anxiety.

As heatwaves become more frequent and severe as a result of climate change, doctors and health organisations are warning that some of the most commonly taken drugs – including psychiatric, blood pressure and heart medications – can interfere with the body’s ability to regulate its temperature, and are putting people at risk of heat-related illness.

Mental health drugs in the heat

Many types of drugs used to treat mental health conditions, including depression, anxiety, and bipolar disorder, are known to increase heat vulnerability due to how they interact with the hypothalamus, the brain’s thermoregulation centre.

“This area of the brain gets us to reduce our activity, to sweat more, to reduce our metabolism – all beneficial adaptations to heat,” says Robert Feder, a retired psychiatrist based in New Hampshire and member of the steering committee for the Medical Society Consortium on Climate and Health.

That includes the most widely prescribed antidepressants, SSRIs (selective serotonin reuptake inhibitors) and SNRIs (serotonin and norepinephrine reuptake inhibitors). Drugs in these classes include sertraline, fluoxetine, escitalopram and venlafaxine. They function by blocking the brain’s reuptake of serotonin, a naturally occurring chemical that regulates mood and other bodily functions.

Too much serotonin in the hypothalamus can disrupt thermoregulation and cause excessive sweating. This affects around one in five people taking antidepressants and can lead to dehydration and electrolyte loss.

“That becomes a compounded problem when you’re already dehydrated because of high temperatures,” says Feder.

Tricyclic antidepressants, an older type of antidepressant, can also trigger sweating – or suppress it instead, also a problem since it reduces heat elimination and raises body temperature. These antidepressants have an “anticholinergic” effect, meaning they block activity of a neurotransmitter called acetylcholine, which is involved in many different bodily functions including sweat production.

The combined effects of taking three different psychiatric medications – an antidepressant, a mood stabiliser and ADHD medication – have taken a toll this summer on Caroline Fraissinet, a 36-year-old career coach in Los Angeles. In the middle of an unusually long, humid heatwave in July, she found herself losing focus, feeling more irritable and sweating more than usual.

At first, she put her symptoms down to poor diet and not drinking enough water. Then a friend from her mental health support group sent her articles connecting heat and medication, and others in the group said they had noticed the same challenges. “As well informed as I am about my mental health conditions and the side effects [of my medications], this was one I just didn’t know,” she says.

Other types of psychiatric medicines can also have an impact. Antipsychotics, used to treat schizophrenia and bipolar disorder, block dopamine in the hypothalamus, which prevents normal responses that would cool the body. Studies of previous heatwaves have linked antipsychotic use to a higher chance of mortality. Stimulants used to treat ADHD, like Ritalin, increase heat production and cause constriction of the blood vessels, which disrupts the body’s natural cooling process. They can also mask exertion, meaning people may not realise they are overheating, says Feder.

From beta blockers to diuretics

But it’s not just psychiatric medicines that can increase vulnerability to heatwaves. Blood pressure and heart medicines are another group of drugs that affect heat sensitivity.

Diuretics, ACE (angiotensin-converting enzyme) inhibitors and ARBs (angiotensin II receptor blockers), commonly used to treat high blood pressure, can increase the risk of electrolyte imbalances. The latter two can also interfere with the hormone that normally triggers thirst.

“With that combined impact, you can get really severely dehydrated to the point where it increases your risk of renal failure, a heart attack, or one of many other severe organ impacts,” says Logan Harper, a family physician and assistant fellowship director of the University of Colorado Anschutz’s climate and health programme.

Beta blockers, meanwhile, a common cardiac medicine, can limit blood flow to the skin, meaning the body can’t release heat as effectively. And besides tricyclic antidepressants, the other types of anticholinergic drugs – used to treat a wide range of conditions like Parkinson’s disease, overactive bladder, respiratory disorders and allergies – can all affect the body’s sweating and cooling mechanisms.

“A lot of times people are on multiple medications that can both interrupt their ability to cool down and increase their susceptibility to organ damage,” says Harper. This is especially the case for older adults, who are also more prone to heat illness in general, he adds.

Even milder heat could be being made more of an issue by some medicines: a study of older patients in the US taking medications, including diuretics, antipsychotics, beta blockers, stimulants and anti-hypertensives, concluded that older patients may be at heightened risk for heat-related hospitalisation in the summer even in the absence of heatwaves. People over the age of 65 are also more likely to be taking multiple medications.

“It’s a given that combinations of those medicines have increased risk beyond just taking one at a time,” says Harper. However, it’s not yet well understood which drug combinations pose the greatest danger, he adds.

Socioeconomic factors also affect risk. “Someone might have a manual labour job that exposes them to a lot of heat, or they might even live outside,” says Harper. “Or maybe you live in an urban heat island, and you can’t afford air conditioning.” (Urban heat islands are densely populated areas with few trees and large areas of dark concrete that absorb heat.)

Cecilia Ahanonu, an emergency medicine physician assistant and assistant professor at Touro University in southern Nevada, recently witnessed this firsthand. During a community health outreach event in Las Vegas, when temperatures were around 43C [109C], she met a man in his 60s who was sleeping in a park. “He looked pale, was sweating profusely, and had a high heart rate,” she says.

The man, who was taking a diuretic and an SSRI, didn’t realise how overheated he was. “He did what most any of us would do and attributed his symptoms to the extreme heat,” says Ahanonu. But as she asked more questions, another possible cause emerged: his medication.

Ahanonu advised him on basic heat safety – drinking more water, avoiding the Sun during peak hours – and gave him a list of nearby cooling centres. She also warned him about more serious symptoms to watch out for which could indicate heat stroke – a potential medical emergency.

“Any altered mental state or confusion, or if someone’s not acting like themselves, you would want to take the person immediately to the nearest emergency room,” she says.

A better awareness 

Doctors caution against stopping medicines without first seeking medical advice. Harper says he would like to see better training and awareness of the heat and medicines’ connection among doctors themselves.

“I think every time a physician prescribes a new medication, they should take a minute to list some of the highest risk side effects,” he says. “There are certain medications, especially in high-risk populations, where heat-related illness should be included in that.”

For Okumus, her experience this summer has made her rethink whether she even wants to be on medication at all. “I started thinking more seriously about going to my doctor and dropping it completely because I can’t work or live my life like this during summer,” she says.

Fraissinet, though, hasn’t considered coming off her medicine. “The medication works really well for me,” she says. Just knowing that this is a side effect of her medicines, rather than something else physically wrong, has put her at ease, she says.

“It signals to me, OK, this is a real thing – rather than just a random occurrence,” she says. “That in and of itself has helped a lot.”