Don’t get in the car after taking it! Serious Risks Lurking in “Hay Fever Medication” in Pharmacies [Urgent Warning from Medical Practitioners and Experts].

The body may feel better, but the ability to avoid danger is diminished

It is no exaggeration to say that hay fever is now a national disease. In early spring, when the arrival of cedar pollen reaches its peak, not a day goes by without seeing a commercial for hay fever medication on TV, but few people know that hay fever medication carries serious risks.

The Pharmaceuticals and Medical Devices Agency (PMDA) has received numerous reports of adverse events such as somnolence, loss of consciousness, and traffic accidents suspected to be caused by the side effects of hay fever medication – antihistamines.

When pollen invades the body, histamine is released from immune cells, which binds to histamine receptors in the body, causing allergic reactions such as itching.

Antihistamines suppress allergic reactions by blocking this binding, but histamine is also a neurotransmitter necessary for the brain to stay awake.

Manabu Tashiro, a professor at Tohoku University’s Research Center for Advanced Quantum Beam Science, who has been studying antihistamine-induced brain dysfunction, says, “Histamine in the brain is a neurotransmitter.

Histamine activates neurons in the brain to keep people ‘awake. Sedating antihistamines suppress nerve activity, increasing the risk of drowsiness and decreased alertness.

Sedating antihistamines are an older type of antihistamine, the first generation, and are characterized by their ease of entry into the brain.

Tashiro joined the research group of Kazuhiko Taniuchi (now a professor at Sendai Shirayuri Women’s University), who had been studying histamine at Tohoku University since the 1980s, and used positron emission tomography (PET) to measure how well antihistamines block histamine binding. Positron emission tomography (PET) to measure how much histamine binding is blocked by antihistamines.

We found that when 10% of histamine receptors in the brain are occupied by antihistamines, cognitive function is affected. The effect is stronger in proportion to the dose of the drug. We also confirmed that the older the antihistamine, the higher the occupancy rate,” said Taniuchi and Tashiro.

The graph shows the occupancy rates of antihistamines and histamine receptors measured by the two researchers. It is important to check which hay fever medication you are taking and understand the risk of cognitive decline.

An experiment on the effects of drugs on the ability to drive a car showed frightening results.

Drowsiness and reduced alertness are probably the greatest concerns when operating machinery. In Europe, the “effects of drugs on the ability to drive a car” have long been studied, but “few people in Japan were doing it,” said Tashiro and Taniuchi, who conducted an experiment on a real car.

The subjects, 18 male adults, were asked to “step on the brake as the lights went on” while driving a 3-km course. The following three drugs were administered at intervals of at least one week. Measurements were taken at three separate times: before the medication, 90 minutes after the medication, and 240 minutes after the medication.

Sedating antihistamine (hydroxyzine 30 mg, occupancy approximately 70%)
Non-sedating antihistamine (fexofenadine 120 mg, <5%)
Fake drugs (placebo)

‘When changes in driving ability were assessed by measuring brake reaction time, it was found that hazard avoidance was significantly reduced when sedating antihistamines were taken, including a delay in the timing of brake application.

Notably, the combination of ‘sedating antihistamines’ and ‘cell phone calls. Even hands-free calls increased the delay by a factor of 2 to 3. Some subjects ran 6.6 meters (equivalent to 60 km/h) longer before hitting the brakes.”

A study using computerized testing identified instances in which performance on cognitive function tests declined significantly after taking antihistamines.

Even in subjects who did not feel sleepy, their reaction times, attention span, and work capacity declined. Even if the subject feels ‘not sleepy,’ it is possible that brain performance is impaired,” said Tashiro.

Our research and studies in the U.S. have shown that 2 mg of chlorpheniramine causes a decline in concentration, judgment, and work capacity similar to a hangover, which is equivalent to drinking three glasses of whiskey,” says Taniuchi.

Eye drops are a blind spot. Over-the-counter eye drops for hay fever contain chlorpheniramine, which “can increase the histamine receptor occupancy rate to as high as 60% to 70%,” Taniuchi warns.

Eye drops do not pass through the liver (are not detoxified) and enter the bloodstream, so they can easily migrate to the brain. Eye drops are not necessarily safe because they are eye drops, so use non-sedating antihistamine-containing eye drops prescribed by a physician. While antihistamines are necessary, please take safe precautions, such as refraining from driving on the day you take them.”

Under the Criminal Code, driving under the influence of drugs is a dangerous driving offense, and many antihistamines are designated as “driving prohibited drugs. It is important to keep in mind that “I’m not sleepy, so I’m fine” is not acceptable.

From the March 27/April 3, 2026 issue of “FRIDAY

Kazuhiko Taniuchi, a leading histamine researcher, graduated from Tohoku University School of Medicine in 1981, received his medical license, and completed the Tohoku University Graduate School of Medicine in 1986, receiving his M.D. degree. He studied at Johns Hopkins University in the U.S. from 1986 to 1987, became an assistant professor at Tohoku University School of Medicine in 1988, and a professor in 1998, retiring from Tohoku University in 2022. He is currently a specially appointed professor at Sendai Shirayuri Women’s University.

Manabu Tashiro, a well-known researcher in the fields of dementia and functional brain imaging, graduated from Shinshu University School of Medicine in 1994 and received his medical license. From 2024, he will become a professor at the Center for Advanced Quantum Beam Science, Tohoku University, as part of the reorganization of the university. In 2024, he became a professor at the Center for Advanced Quantum Beam Science, Tohoku University, where he is involved in research on brain functions, including dementia.