The two largest studies point opposite ways on dementia
Among 284,343 English patients, antihistamines alone showed no significant link at the heaviest exposure. Among 714,052 Taiwanese patients, the sedating ones rose to 51% higher.
What the study found
The two largest studies disagree about antihistamines and dementia
Both dwarf the study usually quoted. Among 284,343 English patients, antihistamines alone showed no significant link at the heaviest exposure: 14% higher odds, on a range from 2% lower to 34% higher. Among 714,052 Taiwanese patients, the sedating ones rose steadily to 51% higher.
The caveat · Drug not split from why it was taken
The English null is wide: a real 20 to 30% increase sits inside its range. The Taiwanese comparison group is small, 36,081 non-users against 677,971 users, and people with hay fever who take nothing are unlikely to resemble those who do. Neither design separates the drug from the reason it was taken.
What we don't know
Whether antihistamines raise dementia risk is unsettled. English prescription records show no significant link, Taiwanese dispensing records a rise with dose, and English records miss most shop-bought use, so neither can correct the other.