Don’t Drive After Taking It Serious Risks Hidden in Over-the-Counter Hay Fever Medications Warn Doctors and Experts

Even if the body feels better, risk-avoidance ability declines

Hay fever, now so common it could be called a national disease, peaks in early spring when cedar pollen is in the air. During this season, TV commercials for hay fever medicine are everywhere—but many people are unaware of the significant risks these medications can carry.

The Pharmaceuticals and Medical Devices Agency (PMDA) has received numerous reports of adverse events suspected to be caused by side effects of antihistamines used for hay fever, including drowsiness, loss of consciousness, and traffic accidents.

When pollen enters the body, immune cells release histamine, which binds to histamine receptors in the body, triggering allergic reactions such as itching. Antihistamines block this binding to suppress allergic reactions—but histamine is also a neurotransmitter essential for maintaining wakefulness in the brain.

Professor Manabu Tashiro of Tohoku University’s Advanced Quantum Beam Science Research Center, who studies cognitive decline caused by antihistamines, explains:

“Histamine activates nerve cells in the brain, keeping a person awake. When sedating antihistamines enter the brain, nerve activity is suppressed, increasing the risk of drowsiness and reduced attention.”

Sedating antihistamines are the first-generation, older type that easily cross the blood-brain barrier.

Tashiro participated in research led by Kazuhiko Taniuchi (currently Specially Appointed Professor at Sendai Shirayuri Women’s University), who has studied histamine at Tohoku University since the 1980s. Using PET scans, they measured how much antihistamines block histamine binding.

“We found that if 10% of histamine receptors in the brain are occupied by antihistamines, cognitive function is affected. The effect grows stronger with higher doses, and older types of antihistamines have higher occupancy rates,” they reported.

The graph shows the occupancy rate of antihistamines at histamine receptors measured by Tashiro and Taniuchi. It is important to check which drug your hay fever medication contains to understand the risk of cognitive decline.

Terrifying results from experiments on the effects of medication on driving ability

The biggest concern with drowsiness and reduced attention is operating machinery, especially driving. In Europe, the effects of medication on driving ability have long been studied, but according to Professors Manabu Tashiro and Kazuhiko Taniuchi, almost no one did such studies in Japan, so they conducted real-vehicle experiments themselves.

The study involved 18 adult men driving a 3 km course while performing a task: pressing the brake when a signal lamp lit up. Participants took three types of medication at intervals of over a week, and their performance was measured at three points: before taking the drug, 90 minutes after, and 240 minutes after. The medications were:

・Sedating antihistamine (hydroxyzine 30 mg, ~70% receptor occupancy)

・Non-sedating antihistamine (fexofenadine 120 mg, <5% receptor occupancy)

・Placebo

“When we measured braking reaction times to assess driving ability, we found that sedating antihistamines significantly delayed the timing of braking, indicating reduced hazard-avoidance ability,” the researchers reported.

A particularly notable combination was sedating antihistamines plus mobile phone calls. Even hands-free calls increased reaction delay by 2–3 times. Some participants drove an extra 6.6 meters before braking (equivalent to 60 km/h).

In computer-based tests, cognitive performance also dropped significantly after taking antihistamines.

“Even participants who didn’t feel sleepy showed decreased reaction time, attention, and task ability. Even if they felt not sleepy, brain performance could still be impaired,” said Tashiro.

“Our research and U.S. studies show that taking 2 mg of chlorpheniramine can reduce concentration, judgment, and work ability to a level comparable to a hangover from three glasses of whiskey,” Taniuchi added.

A blind spot is eye drops. Some over-the-counter antihistamine eye drops for hay fever contain chlorpheniramine, and Taniuchi warns that histamine receptor occupancy can reach 60–70%, which is high.

“Eye drops bypass the liver and enter the bloodstream directly, making it easier for the drug to reach the brain. Just because it’s an eye drop doesn’t mean it’s safe. Use non-sedating antihistamine eye drops prescribed by a doctor. Antihistamines are necessary medication, but avoid driving on the day you take them and take other safety measures.”

Under Japanese law, driving under the influence of medication can be prosecuted as dangerous driving causing injury or death, and many antihistamines are classified as driving-prohibited drugs. The assumption of “I’m not sleepy, so it’s okay” does not apply.

From “FRIDAY”, March 27–April 3, 2026 combined issue

Kazuhiko Taniuchi – Leading researcher on histamine. Graduated from Tohoku University School of Medicine in 1981 and obtained a medical license. Completed graduate studies in 1986, earning a PhD in medicine. Studied at Johns Hopkins University in 1986–1987. Became assistant professor at Tohoku University in 1988 and full professor in 1998. Retired in 2022, now Specially Appointed Professor at Sendai Shirayuri Women’s University.

Manabu Tashiro – Known for research in dementia and brain function imaging. Graduated from Shinshu University School of Medicine in 1994, received medical license. Completed doctoral studies at Tohoku University in 2000. Studied at University of Freiburg, Germany in 2000–2001. Joined Tohoku University faculty in 2001, became professor in 2012, and since 2024 has been a professor at the Center for Advanced Quantum Beam Science at Tohoku University, conducting research on brain function including dementia.