5 Prescription Drugs That Hinder Heat Tolerance

As temperatures rise, so does the risk of heat-related illness - especially for people taking certain prescription drugs.

The body uses several mechanisms to regulate temperature: sweating, blood flow to the skin and fluid balance. But some commonly prescribed drugs interfere with these processes, making it harder to stay cool.

Here are a few you should know about this summer:

1. Antidepressants

Two specific types of antidepressants - selective serotonin reuptake inhibitors (SSRIs) and tricyclics (TCAs) - may make summer harder to cope with. They could even cause heat intolerance due to how they affect the body's ability to sweat.

Both these antidepressants are believed to partly work by affecting neurotransmitter levels in the brain - primarily serotonin and noradrenaline. However, they may also affect other neurotransmitters.

For instance, TCAs can block acetylcholine , a neurotransmitter important for sweating. This may cause some patients to sweat less . On a hot day, this could make it hard to cool down.

However, TCAs also increase noradrenaline levels, a neurotransmitter that stimulates sweat glands. This can increase sweating. In fact, both SSRIs and TCAs can increase sweating in users. Data shows up to 14% of antidepressant users experience this side-effect.

SSRIs may also interfere with the hypothalamus - the brain region that controls body temperature and tells the sweat glands to start producing sweat. But this signal may be affected by the increased serotonin levels.

Since sweating is a key cooling mechanism, any disruptions to this process can lead to heat-related illness. Excess sweating may also cause dehydration if fluids aren't replaced.

2. Antipsychotics

Antipsychotics are used to treat psychosis, which can occur in schizophrenia and bipolar disorder. They do this by blocking the neurotransmitter dopamine - which in turn affects levels of the neurotransmitter serotonin. This can disrupt the hypothalamus's ability to sense and respond to body temperature changes.

As a result, someone taking an antipsychotic might not feel overheated or thirsty when it's hot out. This can cause low blood pressure and reduced heart function . The body then tries to compensate by narrowing the blood vessels and holding in heat. This in turn reduces sweating and makes it harder to cool down properly.

Additionally, antipsychotics have anticholinergic properties . This means they block the action of acetylcholine, making it harder to sweat.

3. Heart medications

Beta-blockers are used to manage heart failure and arrhythmias. They do this by lowering heart rate and reducing how forcefully the heart pumps. But this can limit blood flow to the skin - making it harder for the body to release heat on hot days.

Diuretics are also commonly used in the treatment of high blood pressure or heart failure. But as these drugs increase urine output , this may lead to dehydration and electrolyte imbalances during hot weather. With less fluid available, the body may struggle to sweat properly. If you become severely dehydrated , you might actually stop feeling thirsty. Blood pressure can also drop, which may cause dizziness or fainting - especially when standing up.

Ramipril and losartan, also used to manage blood pressure, can increase the risk of dehydration as well. These drugs block a system in the body that helps control blood pressure, fluid balance and thirst. This may reduce your natural urge to drink, increasing dehydration risk when it's hot.

4. Stimulants

Stimulants - such as the amphetamines used for ADHD - affect many brain chemicals, including dopamine and noradrenaline. This can increase body temperature, boost metabolism and change how the body sweats - all of which can make it harder to cool down , especially when exercising or in hot weather. This can also potentially lead to dehydration, overheating or even heatstroke. Stimulants may also reduce the feeling of tiredness, which can cause people to overexert themselves without realising the danger.

However, some recent research shows people with ADHD who take stimulants may actually have a lower chance of heat-related illnesses - but larger trials are needed to investigate further. The researchers hypothesise that this protective effect may be due to factors such as lower body weight and users staying hydrated.

5. Insulin

Warm temperatures cause the body's blood vessels to dilate (widen) in order to help us cool off. But this action means insulin is absorbed into the bloodstream faster as there's enhanced blood flow to the area where insulin is injected - making blood sugar drop more quickly. This can lead to hypoglycemia (low blood glucose level), which may lead to dizziness, shaking, sweating, irritability and even potentially loss of consciousness or seizures.

Faster insulin absorption can also make it harder for people with diabetes to notice signs of low blood sugar in the heat, since common symptoms may be mistaken for being hot.

Heat can also degrade insulin , reducing its effectiveness and making it unsafe. This is why insulin should be stored in the fridge until use - especially in summer. Damaged insulin will change appearance - turning cloudy or changing colour.

Avoiding heatstroke

Older adults , people with chronic conditions (especially those with heart or lung disease) and those taking multiple prescription drugs are especially vulnerable to heat-related illnesses.

Fortunately, there are steps you can take to stay safe during summer if you're taking one of these common prescription drugs.

First, check labels for storage instructions. Avoid leaving medications in hot places, such as in cars or on windowsills. Insulin isn't the only drug affected by heat - inhalers and EpiPens can also malfunction or become less effective.

Second, stay hydrated when it's hot - unless your doctor has advised otherwise. Dehydration can actually worsen the effects of many medicines. For example, anti-inflammatory painkillers (such as ibuprofen) are more likely to cause kidney problems and bipolar medicines (including lithium ) can become toxic if you're dehydrated.

Avoid peak heat hours and stay in cool environments when possible. Watch for warning signs of heat-related illness - such as dizziness, confusion, nausea or excessive sweating.

Last, don't stop taking your prescriptions medication without medical advice. If it's affecting your ability to cope with the heat, speak with your doctor or pharmacist.

Dipa Kamdar does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.