As scorching heat hits the US, roughly 46 million Americans taking certain types of psychiatric medications face a hidden danger.

Summer can be treacherous for Americans taking antidepressants and antipsychotics such as Zoloft, Lexapro and Geodon for schizophrenia or bipolar disorder. They are significantly more vulnerable to heat exhaustion and heat stroke.

The medications, while increasing serotonin or blocking dopamine receptors, respectively, can cause overheating by confusing the brain’s hypothalamus, sometimes referred to as the internal thermostat.

The hypothalamus relies on serotonin and dopamine to sense and regulate body temperature. Drugs that alter these neurotransmitters cause the body to misread temperature signals and fail to trigger sweating when it overheats, which reduces its ability to cool itself properly. 

Temperatures in the coming days are expected to exceed 90 degrees, with highs near 100 in large swathes of the country, including 101 degrees in New York City, 99 in Dallas, Texas, 99 in Raleigh, North Carolina, 93 in Chicago, Illinois, and 95 in Atlanta, Georgia.

And the 'real-feel' temperatures are forecast to climb to nearly 110 degrees. 

During a heatwave, people on these medications may not sweat and can overheat, with body temperatures potentially reaching 106 degrees Fahrenheit, or they can become excessively sweaty, leading to dehydration.

Early warning signs of heat exhaustion while on SSRIs include excessive sweating or no sweating, dizziness or lightheadedness, nausea, headache, muscle cramps, and a rapid heartbeat.

Heat exhaustion differs from heat stroke, which can be life-threatening. In addition to showing signs of heat exhaustion, heat stroke can cause seizures or loss of consciousness.

Heat stroke kills when the body's internal temperature rises so high that it becomes toxic to cells and organs, essentially cooking them from the inside out. It is a medical emergency that can be deadly if not treated quickly.

For someone already taking an SSRI, certain everyday choices and personal factors can dramatically increase the risk of heat-related illness.

Exercising outdoors is a major trigger. 

The body is already fighting to cool itself when someone works outside in the heat. Adding an SSRI further compromises the brain’s temperature regulator.

Even young, fit people can quickly find themselves in trouble if they overdo it on a hot day.

Alcohol and caffeine make matters worse. Alcohol also dulls one’s judgment, making them less likely to notice the warning signs.

Both are diuretics, which means they flush fluids from the body and speed up dehydration.

When someone is already struggling to regulate their temperature, these drinks cause a shift from discomfort to medical emergency.

Other medications can compound the risk. Blood pressure drugs and diuretics affect fluid balance.

Antihistamines for allergies can reduce sweating, the body's natural cooling system, while stimulants for ADHD can raise the core temperature and heart rate.

Taking any of these alongside an SSRI multiplies the danger.

Age is another crucial factor. Older adults are more vulnerable because their temperature regulation declines with age and their thirst reflex weakens. Many are also on multiple medications.

Young children are equally at risk because their bodies struggle to adapt to extreme heat and they depend on others to spot the signs.

High doses of SSRIs generally mean higher risk. More serotonin activity means a stronger effect on the brain's thermostat.

Patients on maximum doses should be especially cautious during heatwaves and speak with their doctor about whether any adjustments are needed.

Hydration is key. Small sips throughout the day are better than large amounts at once. When sweating heavily, electrolyte drinks are more effective than plain water at replacing lost salts.

Taking SSRIs in the evening means peak effects on temperature regulation occur while resting, while taking them with food may ease some side effects.

Cooling tactics are essential. Cold compresses on the neck, wrists, and temples can help lower core temperature. Fans work best when combined with a water mist, while cool showers are also effective.

Outdoor activity should be avoided between 10 am and 4 pm, when the sun is strongest.

When going outside, wear loose-fitting, light-colored garments made from breathable fabrics like cotton or linen that allow sweat to evaporate. Wide-brimmed hats can also provide additional protection from direct sun.

Gradually increasing time spent outdoors over several days allows the body to adapt, though this does not entirely eliminate the risk for those on SSRIs or antipsychotics.

Experts stress that no one should stop or skip SSRI doses without medical supervision.

Withdrawal symptoms can include dizziness, nausea and mood swings. More seriously, sudden cessation can trigger a relapse of depression or anxiety, or lead to suicidal thoughts.

Never adjust dosage to manage heat, they say. Any change must be discussed with a prescribing physician, who can weigh the risks and benefits.

Do not ignore the warning signs. Dizziness, nausea, headache and changes in sweating should not be brushed aside.

Pushing through these symptoms can lead to heat exhaustion or heat stroke. If symptoms appear, the person should move to a cool place, drink water and seek medical help if they do not improve quickly.