The Hidden Heatwave Danger:
Pills That Cause Dehydration

When a heatwave strikes, public health officials tell us to drink water and stay indoors. But they rarely mention that the pill you take every morning with breakfast could be short-circuiting your body's natural cooling system.

This is not a fringe concern. The CDC publishes dedicated guidance on heat and medications, noting that commonly prescribed drugs including diuretics, anticholinergic agents, and some psychotropic medications increase the risk of heat-related illness — and that certain combinations, such as an ACE inhibitor or ARB taken with a diuretic, may raise that risk significantly.

1. How Medications React to Heat

That fourth mechanism is the one people never hear about, and it deserves its own warning.

2. The Medication Classes Most Affected

• 1. Diuretics (Water Pills)

  Often prescribed for high blood pressure and heart failure (e.g., Furosemide, Hydrochlorothiazide). They literally force your body to flush out sodium and water. During a heatwave, taking these without drastically increasing your water intake is a fast track to severe dehydration.

• 2. Beta-Blockers & ACE Inhibitors

  These blood pressure medications reduce the blood flow to your skin, preventing your body from releasing heat effectively. ACE inhibitors also suppress your sensation of thirst.

• 3. Antidepressants (tricyclics and SSRIs)

  These two classes affect sweating in opposite directions. The CDC notes that tricyclics decrease sweating through anticholinergic effects — removing the mechanism your body relies on most to shed heat — while SSRIs and SNRIs increase it, which drives fluid loss instead. SSRIs also carry a risk of hyponatremia (low blood sodium), which compounds in hot weather when heavy sweating is paired with large volumes of plain water. Confusion, headache, and nausea in the heat are worth taking seriously on these drugs.

• 4. Antipsychotics

  The CDC lists antipsychotics, anticholinergics, and stimulants as interfering with central thermoregulation — the brain's thermostat — and separately notes that typical and atypical antipsychotics decrease sweating. Drugs in this group also appear on the CDC's sedation and electrolyte-imbalance lists, meaning several mechanisms stack at once. People taking them may not notice they are overheating.

• 5. Stimulants

  Amphetamine-based ADHD medications and similar stimulants increase heat production and physical activity while reducing the sense of exertion. The combination matters most during outdoor exercise in high temperatures.

• 6. Photosensitising drugs (a much longer list than antibiotics)

  The CDC names tetracyclines, fluoroquinolones, metronidazole, and antifungals such as flucytosine, griseofulvin, and voriconazole as drugs that increase sun sensitivity and can cause a sunburn-like rash. Thiazide diuretics such as hydrochlorothiazide, amiodarone, some NSAIDs, sulfonamides, retinoids, and St John's wort are also commonly implicated — and note that hydrochlorothiazide appears on both this list and the dehydration list. The CDC's advice for these medications is sun avoidance, protective clothing and hats, and broad-spectrum sunscreen filtering UVA and UVB at SPF 30 or higher.

• 7. OTC antihistamines & decongestants

  Over-the-counter allergy medicines — diphenhydramine in particular — have anticholinergic effects that decrease sweat production, and decongestants such as pseudoephedrine constrict blood vessels. Both trap heat. These are easy to overlook precisely because no prescription was involved.

Risk is cumulative. Someone on a thiazide diuretic, an antidepressant, and an occasional antihistamine is carrying three separate mechanisms at once — which is far more relevant than any single drug on this list.

3. Heat Damages the Medication Itself

The other half of the problem is storage. Most solid oral medications are meant to be kept at controlled room temperature — the USP target is 20–25°C (68–77°F), with brief excursions permitted between 15–30°C (59–86°F) — and heat degrades many of them silently, with no change you can see. The FDA's guidance on drugs exposed to extreme conditions is blunt: if you think medicines have been exposed to excessive heat, consider replacing them.

If you are travelling somewhere hot, our guide to traveling with medication covers storage in transit and how to shift dose times across time zones.

4. Warning Signs That Need Emergency Care

Heat exhaustion can progress to heat stroke, which is a medical emergency. On the medications above, the usual warning signs can arrive earlier or be masked. The symptom lists below follow the CDC's heat-related illnesses guidance.

Confusion is the sign people dismiss most often, and on antipsychotics or anticholinergic drugs it may appear before the skin feels obviously hot. Treat altered behaviour in the heat as an emergency, not as tiredness.