Hot weather places additional demands on the human body. To prevent its core temperature from rising, the body increases blood flow toward the skin and produces sweat, allowing heat to escape into the environment. At the same time, fluids and electrolytes are lost, blood pressure may fall, and the heart and kidneys must work harder to maintain circulation.
Most people adapt to these changes without developing serious problems. Certain medications, however, can influence sweating, thirst, blood pressure, kidney function, alertness or fluid balance. This does not mean that everyone taking these medicines will become ill during a heatwave, nor does it mean that the medication should be stopped. It means that some people may need additional precautions and an individualized plan for very hot days.
According to the CDC’s clinical guidance on heat and medications, commonly prescribed medicines that may increase vulnerability to heat include diuretics, anticholinergic agents and certain psychiatric medications. The overall risk depends not only on the medicine itself but also on the person’s age, health, level of heat exposure, access to cooling and use of other medications.
How does the body normally cope with heat?
When the temperature rises, blood vessels near the skin dilate. This allows more warm blood to reach the body’s surface, where heat can be released. Sweat glands also produce moisture that cools the skin as it evaporates.
These mechanisms require adequate circulation and access to fluids. If a person becomes dehydrated, blood volume decreases and the body may find it more difficult to maintain blood pressure and transfer heat toward the skin. High humidity creates an additional problem because sweat evaporates less efficiently, even when the body continues producing it.
Prolonged heat exposure can therefore place considerable strain on the cardiovascular and renal systems. The World Health Organization notes that extreme heat can worsen cardiovascular disease, diabetes, kidney disease, respiratory illness and certain mental health conditions. Medications prescribed for these conditions may sometimes add another layer of vulnerability.
Medications can affect heat tolerance in different ways
There is no single interaction between medication and heat. Different medicines can influence the body through different mechanisms. Some increase fluid loss through urination, while others reduce the sensation of thirst. Certain drugs interfere with sweating or with the dilation of blood vessels near the skin, making it more difficult to release heat.
Other medicines may lower blood pressure, cause dizziness or reduce alertness. During a heatwave, these effects can become more significant because heat itself can contribute to lower blood pressure, fatigue and dehydration. Sedating medications may also make it harder for a person to recognize early symptoms, obtain fluids or move to a cooler environment.
Dehydration can in turn affect the way medicines are processed by the body. Reduced blood flow to the kidneys may slow the elimination of certain drugs, potentially increasing their concentration. This is particularly important for medicines with a narrow therapeutic range, such as lithium, for which relatively small changes in blood concentration can become clinically significant.
Diuretics and fluid loss
Diuretics, sometimes called “water tablets”, increase the amount of water and sodium removed through the urine. They are commonly prescribed for high blood pressure, heart failure, oedema and certain kidney or liver conditions.
During hot weather, the body is already losing water and electrolytes through sweat. The additional urinary losses caused by a diuretic may increase the likelihood of dehydration, low blood pressure or electrolyte imbalance, especially when fluid intake is inadequate or the person is also experiencing vomiting or diarrhoea.
The risk may be greater when a diuretic is combined with an angiotensin-converting enzyme inhibitor, known as an ACE inhibitor, or an angiotensin receptor blocker, known as an ARB. These combinations are medically necessary for many people, but the CDC advises clinicians to consider their combined effects when creating a plan for periods of extreme heat.
Warning signs can include unusual thirst, dry mouth, dizziness when standing, weakness, confusion, muscle cramps, reduced urination or urine that is darker than usual. These symptoms are not specific to medication-related dehydration, but they should not be ignored during prolonged heat.
Blood pressure and cardiovascular medicines
Several cardiovascular medicines can influence the way the body responds to heat. ACE inhibitors and ARBs may reduce blood pressure and, in some people, lessen the normal sensation of thirst. Beta-blockers can limit the increase in heart rate and may reduce blood flow toward the skin, while calcium-channel blockers and nitrates can contribute to lower blood pressure.
This does not make these medicines unsafe in summer. Their benefits remain essential for controlling cardiovascular disease. The concern arises when the effects of the medication combine with dehydration, prolonged standing, heavy sweating or sudden exposure to extreme temperatures.
A person whose blood pressure is normally well controlled may experience light-headedness, weakness or an increased risk of falling during a heatwave. Older adults and people taking several blood pressure medicines may be especially vulnerable. Any adjustment, however, must be made by the treating physician rather than by skipping doses independently.
Anticholinergic medicines and certain antihistamines
Anticholinergic effects occur in a wide range of medicines, including treatments for bladder problems, motion sickness, nausea, Parkinson’s disease and certain respiratory or psychiatric conditions. Some older, sedating antihistamines also have substantial anticholinergic activity.
These medicines can reduce sweating. Because the evaporation of sweat is one of the body’s main cooling mechanisms, reduced sweat production can make it more difficult to release heat. Anticholinergic effects may also cause dry mouth, blurred vision, constipation, urinary retention, drowsiness or confusion.
Not every antihistamine has the same effect. The risk is more closely associated with antihistamines that have sedating and anticholinergic properties, rather than with the entire medication category. People who regularly use an antihistamine or an over-the-counter sleep product should therefore check its active ingredient and ask a pharmacist whether it may affect heat tolerance.
Psychiatric medications
Some antipsychotic medications can interfere with the brain’s regulation of body temperature and may also impair sweating. Sedation or reduced awareness can make the situation more difficult by delaying recognition of thirst, dizziness or overheating.
Antidepressants do not all affect heat tolerance in the same way. Tricyclic antidepressants can decrease sweating because of their anticholinergic effects. Selective serotonin reuptake inhibitors and serotonin-norepinephrine reuptake inhibitors may increase sweating, potentially contributing to greater fluid loss under certain conditions.
Lithium requires particular attention. Dehydration and changes in sodium balance can raise lithium concentrations and increase the risk of toxicity. Symptoms such as worsening tremor, marked drowsiness, vomiting, diarrhoea, muscle weakness, poor coordination or confusion require prompt medical advice.
Psychiatric medication should never be stopped suddenly because of hot weather. Abrupt discontinuation may cause withdrawal effects, recurrence of symptoms or other serious complications. A pre-arranged heat plan with the prescribing doctor is much safer than making changes during a heatwave without guidance.
Stimulants and neurological medicines
Stimulants used for attention disorders can increase body temperature and may affect heat perception or circulation. Recreational stimulants, including amphetamines and MDMA, present an additional and potentially severe risk, especially when used during strenuous activity, in crowded venues or in environments with poor ventilation.
Certain neurological medicines can also affect sweating or fluid balance. Topiramate, for example, may reduce sweating, while some antiseizure medicines can contribute to dizziness, weakness, increased sweating or changes in sodium levels.
The degree of risk varies significantly among medicines and individuals. A person taking neurological medication should not assume that every symptom in hot weather is a side effect, but unexplained changes in sweating, alertness, balance or urination deserve medical attention.
Painkillers and other over-the-counter products
Over-the-counter medicines are often overlooked when people review their heat-related risks. Non-steroidal anti-inflammatory drugs, such as ibuprofen and naproxen, can place additional strain on the kidneys when a person is dehydrated. The risk is more significant in older adults, people with kidney or heart disease and those taking diuretics or certain blood pressure medicines.
Laxatives can increase fluid and electrolyte losses, particularly when they cause frequent or loose bowel movements. Sedating sleep aids and some cold or allergy products may contain antihistamines with anticholinergic properties, even when the consumer does not think of them as antihistamine medication.
For this reason, a medication review should include prescription medicines, non-prescription products and supplements. The absence of a prescription does not necessarily mean that a product cannot interact with heat, dehydration or another medicine.
Who is most vulnerable?
The interaction between medication and heat is rarely determined by one factor alone. Risk tends to increase when several vulnerabilities occur together. Older adults may have a reduced thirst response, less efficient thermoregulation and a greater likelihood of taking multiple medicines. Infants and children also regulate body temperature differently and depend on adults to maintain hydration and a safe environment.
People with heart failure, kidney disease, diabetes, dementia, severe mental illness or limited mobility may have greater difficulty adapting to heat. The same applies to people who work outdoors, exercise intensely, live in poorly ventilated homes or do not have reliable access to a cool environment.
A medication that causes no noticeable difficulty during moderate weather may become more relevant when temperatures remain high overnight, humidity rises or a person suddenly travels to a much hotter climate.
Never stop medication without medical guidance
The most important rule is that medication should not be stopped, reduced or rescheduled independently because of a heatwave. For many conditions, missing even a few doses can create a greater risk than continuing treatment.
The CDC recommends individualized plans that consider the medicine, the medical condition, the person’s symptoms and the risks of any adjustment. Such a plan may include guidance about monitoring blood pressure, fluid intake, body weight, blood glucose, kidney function or warning symptoms.
People taking several medications, particularly diuretics, cardiovascular drugs, lithium or medicines with anticholinergic effects, can ask their doctor or pharmacist a direct question: “Do I need a specific medication and hydration plan during periods of extreme heat?”
Hydration advice must also be individualized
Drinking water regularly is generally important during hot weather, but “drink as much as possible” is not universally safe advice. Some people with heart failure, advanced kidney disease or other conditions have been instructed to restrict their fluid intake. Suddenly consuming large quantities of water may be inappropriate and can disturb sodium balance.
People following a fluid restriction should ask their medical team how the plan should be managed during a heatwave. Electrolyte drinks are also not automatically necessary or suitable for everyone, as some contain considerable amounts of sugar, sodium or potassium.
The safest approach is to follow individualized medical guidance, drink at regular intervals when permitted and avoid waiting until intense thirst develops, particularly when taking a medicine that may weaken the thirst response.
Recognizing early warning signs
Early symptoms of heat exhaustion or dehydration may include:
- headache, dizziness or unusual weakness,
- intense thirst or dry mouth,
- nausea or vomiting,
- heavy sweating or an unexpected reduction in sweating,
- muscle cramps,
- reduced or dark-coloured urine,
- rapid heartbeat,
- light-headedness when standing,
- unusual drowsiness or difficulty concentrating.
A person experiencing these symptoms should move to a cooler environment, stop physical activity, loosen unnecessary clothing and begin cooling the body. Fluids may be taken gradually when the person is conscious and has not been medically advised to restrict them. If the symptoms do not improve promptly, medical advice is needed.
Confusion, loss of coordination, seizures, collapse, loss of consciousness or a very high body temperature can indicate heatstroke. Heatstroke is a medical emergency and requires immediate emergency assistance. A person with altered consciousness should not be forced to drink.
Heat can also damage medicines and medical devices
Hot weather does not affect only the person taking the medicine. It can also affect the product itself. Medicines should not be left in parked cars, direct sunlight or bags that become extremely hot.
Insulin, certain liquid medicines, adrenaline auto-injectors, inhalers, hormone patches, blood glucose monitors and test strips may be particularly sensitive to inappropriate temperatures. Storage requirements vary by product, so the instructions in the patient leaflet should always be followed.
The UK Medicines and Healthcare products Regulatory Agency advises consulting a pharmacist if a medicine changes colour, smell, texture or appearance, or if there is concern that it has been exposed to excessive heat. Refrigerated medication should not be placed directly against ice unless the manufacturer’s instructions specifically allow it, as freezing can also damage certain products.
Creating a safer plan for hot days
Preparation is more effective than trying to make medication decisions after symptoms have already appeared. Before or at the beginning of the hot season, people who take regular medication can review their complete medication list with a healthcare professional, including over-the-counter products and supplements.
It is also useful to know which symptoms require medical advice, where medicines should be stored, how to manage them during travel or a power cut and whether a family member should check in during periods of extreme heat. Avoiding strenuous activity during the hottest hours, spending time in a cool environment and paying attention to official heat warnings can further reduce exposure.
Understanding the relationship between medications and heat
Medicines do not affect everyone’s response to hot weather in the same way. Some increase fluid or electrolyte loss, others alter sweating, thirst, circulation or alertness, and dehydration itself can change the way certain drugs are cleared from the body.
The presence of a medicine on a heat-risk list does not mean that it is inappropriate or that treatment should be interrupted. These lists are intended to identify people who may benefit from greater awareness, closer monitoring and an individualized plan.
The safest response to a heatwave is not to change treatment independently, but to understand how the medication works, reduce unnecessary heat exposure, follow appropriate hydration advice and seek professional guidance when symptoms or concerns arise.
