When the temperature rises, most people sweat to cool down. But if you're taking certain medications, your body might not be able to sweat at all - or at least not enough. This isn't just uncomfortable. It's dangerous. Heat intolerance from medications is a real, underrecognized risk that can lead to heat exhaustion or even heat stroke, especially during summer heat waves. And it’s not rare. Around 1 in 7 adults in the U.S. who take prescription drugs are affected. If you or someone you know is on meds and feels unusually hot, dizzy, or dry in warm weather, this is something you need to understand.
How Medications Kill Your Body’s Cooling System
Your body cools itself in two main ways: sweating and blood flow to the skin. When you get hot, your brain tells sweat glands to release fluid, and your blood vessels widen to carry heat to the surface. Medications can block either or both of these processes. Take diuretics, for example. These are often prescribed for high blood pressure or heart failure. Common ones like hydrochlorothiazide and chlorthalidone make you pee more - a lot more. Some people lose 1.5 to 2.5 liters of fluid a day just from these pills. Less fluid means less sweat. Studies show this can cut your sweating capacity by 30% to 50%. You might not even feel thirsty, but your body is running on empty. Then there are beta blockers like metoprolol and propranolol. These slow your heart rate and lower blood pressure, but they also prevent blood vessels in your skin from opening up. Without that extra blood flow, heat gets trapped inside. One study found people on these drugs had a 0.15°C higher core temperature during heat exposure than those not on them. That might not sound like much, but in extreme heat, it’s enough to push you into danger. Anticholinergic drugs - used for overactive bladder, allergies, or even Parkinson’s - are even more risky. They block acetylcholine, the chemical your body uses to trigger sweat glands. Medications like oxybutynin and diphenhydramine (Benadryl) can reduce sweating by 60% to 80%. People on these drugs often feel fine indoors, but step outside on a 30°C day, and they’re at high risk of overheating. Some don’t sweat at all.The Most Dangerous Medications
Not all drugs affect heat tolerance the same way. Some are far more dangerous than others. Antipsychotics like olanzapine, risperidone, and haloperidol are among the worst. They don’t just reduce sweating - they mess with your brain’s internal thermostat. The hypothalamus, which controls body temperature, gets confused. This means you can develop heat stroke even at temperatures most people find comfortable - like 27°C to 32°C (80°F to 90°F). The CDC lists these as the highest-risk category. Patients on these meds have a 3.2% higher chance of hospitalization for every 1°C rise above 25°C compared to the general population. Tricyclic antidepressants (TCAs) like amitriptyline are another hidden danger. They cut sweating by 65% to 75%. But here’s the twist: newer antidepressants like fluoxetine (Prozac) and sertraline (Zoloft) do the opposite. They can make you sweat more, which sounds good - until you realize you’re losing fluids faster than you can replace them. Dehydration still happens. GLP-1 agonists like semaglutide (Ozempic, Wegovy) are newer, popular weight-loss drugs. But they suppress thirst by 40% to 50%. You might drink water, but your brain doesn’t tell you to keep going. People on these meds often wake up dehydrated after a hot night, unaware they’ve been losing fluids all day. And lithium - used for bipolar disorder - is especially tricky. It’s already a narrow therapeutic drug, meaning the difference between a helpful dose and a toxic one is small. Dehydration from heat can spike lithium levels by 25% to 35%. That’s enough to cause tremors, confusion, seizures, or even kidney damage. The CDC specifically warns that lithium users need extra monitoring during heat waves.Who’s at Highest Risk?
It’s not just about the drug. It’s about how many you’re taking. People on three or more heat-affecting medications have up to a 300% higher risk of heat stroke than those on one or none. That’s why older adults are especially vulnerable. Many take multiple prescriptions - for blood pressure, depression, bladder control, and more. The American Geriatrics Society says adults over 65 on five or more medications are at extreme risk. Climate change makes this worse. 2023 was the hottest year on record. In Melbourne, summer temperatures now regularly hit 35°C or higher. And by 2050, heat-related medication complications are projected to rise by 40% to 60% if nothing changes. Even the type of medication matters. Non-selective beta blockers like propranolol cut skin blood flow more than selective ones like metoprolol. Thiazide diuretics are worse than potassium-sparing ones. And combining certain drugs - like a beta blocker with a calcium channel blocker - can make the problem even worse.
Signs You’re Overheating
If you’re on any of these meds, learn the warning signs. They don’t always look like classic heat illness.- Cramps (65% of cases)
- Dizziness or lightheadedness (55%)
- Headache (70%)
- Flushed, dry skin - or no sweating at all (85%)
- Nausea or vomiting (35%)
- Weakness or fatigue
- Rapid heartbeat
What You Can Do
You can’t always stop your meds. But you can manage the risk.- Drink more water. Add 500 to 1000 mL extra per day during hot weather. Don’t wait until you’re thirsty.
- Avoid the sun between 10 a.m. and 4 p.m. That’s when UV radiation peaks and temperatures are highest.
- Use air conditioning. If your home doesn’t have it, go to a mall, library, or cooling center. Even a few hours of cool air helps.
- Check your weight daily. Losing more than 2% of your body weight in a day means you’re dehydrated. Weigh yourself in the morning before eating or drinking.
- Wear cooling gear. Cooling vests can drop your core temperature by 0.5°C to 1°C. They’re especially helpful for people on antipsychotics or anticholinergics.
- Use SPF 30+ sunscreen. Some meds - like calcium channel blockers and ACE inhibitors - make your skin more sensitive to sunburn. Protect yourself.
- Ask your doctor. If you’re on multiple meds, ask if any of them affect heat tolerance. Ask if there’s a safer alternative.
What’s New in 2026
Health systems are starting to catch up. In 2023, Epic Systems added automatic alerts in electronic health records for 14 high-risk medications during summer months. If your doctor prescribes one of them, your chart now flags you as heat-sensitive. The FDA approved the first wearable core temperature monitor, TempTraq, in late 2023. It’s designed for people on heat-affecting drugs. It tracks real-time body temperature and sends alerts if you’re rising into danger zone. Researchers are now testing AI tools that analyze your medication list and predict your personal heat risk. A $2.5 million NIH study launched in September 2024 aims to build a tool that tells you: “Based on your drugs, you’re at high risk above 30°C.” That could change how we manage this problem.Final Thought
Heat intolerance from meds isn’t a myth. It’s a measurable, life-threatening side effect. It doesn’t happen to everyone - but it can happen to you. And if you’re on multiple drugs, especially if you’re over 65, you’re already in a high-risk group. Don’t assume you’re fine because you’ve never had a problem before. Heat is getting worse. Medications are getting more common. And your body’s ability to cool down? That’s fragile. Talk to your doctor. Know your meds. Stay cool. Your life might depend on it.Can drinking more water prevent heat stroke if I’m on diuretics?
Drinking more water helps, but it doesn’t fully fix the problem. Diuretics like hydrochlorothiazide force your kidneys to flush out fluid - even if you drink extra. You’re still losing more than you can replace. The key is to reduce heat exposure, not just increase fluids. Your body needs to sweat to cool down, and diuretics cut that ability by 30-50%. So while hydration is important, it’s not enough on its own.
Are over-the-counter meds like Benadryl really dangerous in the heat?
Yes. Diphenhydramine (Benadryl) is an anticholinergic, which blocks sweat glands. Studies show it can reduce sweating by up to 80%. Even a single dose can make you unable to cool down. Many people take it for allergies or sleep, not realizing it’s a major heat risk. The CDC warns that OTC antihistamines are among the top causes of medication-related heat illness in older adults.
Do all antidepressants cause heat intolerance?
No. Tricyclic antidepressants (TCAs) like amitriptyline reduce sweating by 65-75%. But SSRIs like fluoxetine and sertraline actually increase sweating by 20-30%. That sounds helpful, but it can lead to dehydration if you don’t drink enough. So both types carry risk - just in different ways. TCAs stop you from sweating; SSRIs make you sweat too much and lose fluids.
Why is lithium so risky during heat waves?
Lithium is toxic if your blood levels get too high. Dehydration from heat causes your kidneys to hold onto lithium instead of flushing it out. This can spike lithium levels by 25-35%. Even a small increase can cause tremors, confusion, seizures, or kidney damage. The CDC recommends checking lithium levels during heat waves and avoiding outdoor activity when temperatures exceed 29°C (85°F).
Can I switch to a different medication to avoid heat intolerance?
Sometimes, yes. For example, if you’re on a thiazide diuretic, your doctor might switch you to a potassium-sparing diuretic like spironolactone, which causes less fluid loss. If you’re on an anticholinergic for bladder control, mirabegron is an alternative that doesn’t affect sweating. But not all meds have safe alternatives. Never change your meds without talking to your doctor - the risks of stopping could be worse than heat intolerance.
Do cooling vests really work for people on antipsychotics?
Yes. Cooling vests use phase-change materials or circulating coolants to lower core body temperature by 0.5°C to 1.0°C. For people on antipsychotics, who can’t sweat and have impaired brain temperature control, even a small drop can prevent heat stroke. Catholic Health System in the U.S. now recommends them for high-risk patients during heat advisories. They’re not a cure, but they’re a critical safety tool.
