Is It Dangerous to Take Allergy Medicine Every Day?

My husband’s doctor tells him Zyrtec is being seen to cause Alzheimer’s? Is that true? I’ve been giving Zyrtec to my kids, and I also take it almost daily for bad seasonal allergies!
—Emily

This is an interesting example of a case where, after going down the literature rabbit hole, I see where this claim comes from. But also, it’s a bit like a bad game of telephone. I think your husband’s doctor might have been at the end of the line.

The underlying set of findings that seems to motivate this conclusion is a literature on the relationship between neurological functioning in older adults and the use of anticholinergic drugs. Anticholinergic drugs are medications that work by blocking the neurotransmitter acetylcholine from binding to certain nerve cell receptors. This is a very broad drug class. It includes some antidepressants but also things like Imodium, Dramamine, and various drugs to decrease frequent urination. It also includes Benadryl.

Several papers have linked high usage of drugs in this class to dementia or Alzheimer’s disease in older adults. See, for example, here and here. These papers can be difficult to draw conclusions from. Since the drugs are used to treat a wide range of conditions, including depression and other brain-related conditions, it may be difficult to separate out the role of the medications from the correlation in different mental challenges. This is likely to be especially true in populations of older adults, where there may be many different things happening over time.

Putting aside the claims to causality, it’s hard to take any of these results and draw conclusions about a particular one of the many drugs in this drug class. If you wanted to, say, blame Benadryl for these effects, you’d be hard-pressed to make a compelling case.

I suspect it is this link — between a drug class that includes antihistamines and these brain issues — that is what your husband’s doctor is drawing on.

However, there is a last link, which is the fundamental breakdown in the game of telephone. Zyrtec is not generally considered to be in the high-anticholinergic drug space. So even if one did want to conclude that Benadryl was problematic for dementia in older people — again, I think, a stretch — one would not draw from that a conclusion that Zyrtec was also bad. In fact: there is often a recommendation to switch to Zyrtec or other antihistamines in this class for older adults who typically take Benadryl.