"They say even dogs don't catch summer colds. I've struggled every year, but I never imagined cold medicine could be the problem."
A friend who had canceled our weekend plans at the last minute called me. Suffering from worsening sneezing and runny nose due to the wide temperature swings, she had taken a multi-symptom cold tablet before heading out to her third-grader son's soccer class. That, she said, was the trigger. Not long after she stood on the field watching the lesson, a severe headache and dizziness rushed over her, followed by nausea and trembling muscles throughout her body. The antihistamine ingredient in the cold medicine had blocked perspiration, causing her body temperature to spike rapidly and producing the early symptoms of heatstroke. "I gulped down the sports drink in my hand and barely made it to a nearby pharmacy. Only then did I find out what had caused it," she said with relief.
With Seoul's highs already topping 30 degrees Celsius before June even arrives, the early heat wave is putting the health of patients with chronic conditions on red alert. The Korea Disease Control and Prevention Agency reported its first heat-related death in mid-May since the agency began operating its emergency room surveillance system, and more than 50 cases of heat-related illness occurred over a single weekend.
When sweltering heat persists, the best defense is to release sweat appropriately to lower body temperature and drink enough water to prevent dehydration. The problem is that medications routinely taken—such as antihistamines—can simultaneously undermine both of these defense mechanisms.
Beyond cold medicines, antihistamines are widely used to treat rhinitis, hives, and atopic dermatitis. They are available at pharmacies without a prescription and can also be easily purchased at convenience stores. The reason this ingredient leaves the body vulnerable in hot weather is its "anticholinergic action," which suppresses the secretion of acetylcholine, a neurotransmitter in the body. When temperatures rise, acetylcholine stimulates the sympathetic nervous system to induce sweating and lower body temperature. When this signal is blocked, sweat does not flow even as heat builds up inside the body. While outdoor temperatures climb above 30 degrees, the body's heat-release valve is shut off.
The same applies to medications taken by hypertension patients. Diuretics, which lower blood pressure by reducing blood volume, accelerate dehydration in situations where water is already being lost through sweat. Calcium channel blockers can make body temperature regulation difficult because of their mechanism of dilating peripheral blood vessels, while angiotensin-converting enzyme (ACE) inhibitors can cause electrolyte imbalances that lead to dizziness or rapidly impair kidney function. Hypertension is commonly considered dangerous during cold winters, but summer is no time for complacency either. The risks grow even greater for those taking combination drugs containing multiple antihypertensive ingredients. In fact, in rural areas, it is reported that a considerable number of elderly people who diligently take their blood pressure medications collapse unconscious while working in the fields during heat waves and are rushed to emergency rooms. The drugs themselves are not bad; rather, the combination of extreme heat and medication creates unforeseen risks.
Some medications used to treat depression and schizophrenia also contain ingredients with anticholinergic action that suppress sweating. Conversely, there are drugs that induce excessive sweating and worsen dehydration. This is why patients taking tricyclic antidepressants or thyroid hormone medications find summer particularly difficult. Thyroid hormones work by raising the basal metabolic rate, increasing body heat production—and when outdoor temperatures rise, the heat load doubles. If you find yourself wondering, "Why am I the only one feeling this hot?," it may be worth reviewing the medications you are taking. That said, stopping medication on your own is strictly forbidden. Avoid prolonged exposure to extreme heat, and develop the habit of replenishing fluids adequately even when you do not feel thirsty. Before summer begins in earnest, I recommend asking your doctor or pharmacist how the medications you take might affect your body's temperature regulation.
