Overwhelmed by heat? The culprit may be in your medicine cabinet

(CNN) — When Adelaide Saywell posted on TikTok that SSRIs, a commonly prescribed antidepressant medication, could make people more vulnerable to heat, it went viral and sparked a deluge of comments.

“Wait seriously??? I just started taking it a month ago and am so hot all the time,” said one commenter.

“I have been on sertraline for 8 years and NOBODY told me this,” said another.

Some described serious reactions: “I was almost hospitalized for heat stroke (because) they didn’t warn me.”

Saywell, who herself has been on SSRIs for 12 years, has posted similar warnings every summer for the past three years. The first time she did, the response surprised her. “I had assumed I was one of few people who didn’t know this,” she told CNN.

But now she expects it. It’s all further proof of a huge gap in knowledge about this particular side effect, she said.

Around 1 in 10 people in the US take antidepressants, the most common of which is SSRIs — selective serotonin reuptake inhibitors — which are prescribed for depression, anxiety and panic disorder. These drugs, which include Zoloft and Prozac, can be lifesaving, but they can also stop the body from regulating its temperature properly and decrease heat tolerance.

It’s a worrying side effect doctors and scientists are still trying to understand fully, especially as the world heats up and punishing, prolonged heat waves become more frequent.

SSRIs do enormous good, said Dr. Laurence Wainwright, a lecturer at the University of Oxford who researches the mental health impacts of climate change. They “have been revolutionary in psychiatry,” he told CNN. But, as with any drug, there are a host of side effects. “It’s about getting a cost-benefit ratio right,” he said.

SSRIs work by blocking the brain’s reuptake of serotonin — a natural chemical that regulates bodily functions including mood. By increasing the level of serotonin in the brain, they can help ease symptoms of anxiety and depression.

But as they modulate the brains’ chemicals, SSRIs can also tinker with the body’s ability to keep itself cool.

They can affect the hypothalamus, a small structure in the brain which acts as the body’s thermostat, sensing temperature changes and triggering various processes to keep the body’s internal temperature around 98.6 degrees Fahrenheit, or 37 degrees Celsius.

“When you add more serotonin to the mix, (the hypothalamus) becomes less sensitive,” said Dr. Pope Moseley, a physician and biomedical sciences researcher at Arizona State University. Bathed in serotonin, it becomes less effective at sending out the necessary signals.

This can affect the tools the body uses to cool itself, including sweating – its main cooling mechanism – which transfers heat from the skin to the air as it evaporates. Excessive sweating is a common side effect of SSRIs, which can lead to dehydration and, paradoxically, a rise in body temperature.

In other people, however, SSRIs can cause a reduction in sweating, Moseley said. This is also a problem as their body loses a vital process for dumping heat.

There are other ways SSRIs can put people at risk of dehydration. They can activate a hormone that makes people urinate more and there is evidence they can alter people’s perception of thirst, meaning they don’t drink enough to replace lost fluids, Moseley said.

Dehydration puts people at risk of heat exhaustion and, in particularly severe cases, heat stroke, which is dangerous and potentially deadly.

Scientists and doctors are still trying to unpick the complex relationship between SSRIs and heat. “What really shocks me is how little we actually understand about this,” said Wainwright.

It is partly the reason why medical professionals don’t always talk about heat when they prescribe these anti-depressants. “I don’t think that the medical community is very aware of this being an issue,” Wainwright said, “and I certainly think that patients aren’t either, looking on sites like Reddit around this topic.”

Reddit has become an online gathering place for people to seek advice on SSRIs and heat, and to share symptoms.

“I sweat insanely when I was on an SSRI … humidity was unbearable,” said one Redditor. “Heat is always something I have tolerated very well until this medication,” said another.

Others complained that the side effects sometimes affected their mental health: “I was miserable and had to stop going to the gym because I was getting nauseous and felt overheated all the time.”

While heat sensitivity is one of many side effects listed on information sheets provided with SSRIs, often people taking these drugs won’t read them because it increases their anxiety, Saywell said.

“I also understand of course we should be asking questions,” she added, “but how are we to know what questions to ask?”

Doctors often don’t mention the heat side effect because others are more common, such as weight gain, said Dr. Judith Joseph, a board certified psychiatrist, researcher and medical doctor. But they should, she told CNN, especially since overheating can worsen anxiety, depression and mood.

There are simple measures people can take to protect themselves, she added, including avoiding the outdoors during the hottest part of the day, drinking lots of fluids and avoiding caffeine and alcohol, which can lead to further dehydration. Sleep is also important as there is a very strong relationship between poor sleep and worsening depression and anxiety.

Joseph advises people to keep an eye on weather reports and seek extra support if possible during very hot weather.

People should be alert to signs they may be struggling, such as more frequent urination and excess sweating, Moseley said, all of which are signals to drink more.

Symptoms such as nausea, muscle cramps, a racing pulse and mild dizziness could all suggest heat exhaustion. These are signals to find a cool place, rest, drink fluids and even use ice packs against your skin, Joseph said.

More serious symptoms of overheating include fainting, vomiting, a rapid or erratic heart rate, severe headache and difficulties breathing — potential signs of heat stroke. “This requires you to go to the ER or call 911,” she said.

The ultimate message “isn’t that people shouldn’t take SSRIs,” Joseph added, “but what can people who take them do to protect themselves in the heat.”

For now, Saywell plans to keep posting about SSRIs and heat every summer. And she expects she will continue to receive a flood of comments from people who had no idea about the risk heat could pose to them.

“It’s actually rather sad how little is known about this side effect,” she said, “when it is so common and can be potentially dangerous.”

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