Can Popular Antihistamine (Benadryl) Cause Brain Fog?

Q. Over the past 30 years I have taken Benadryl twice. Both times it caused me memory loss as a side effect.

I had to look up phone numbers that I dialed daily. I wasn’t even sure what year it was. Thankfully, the problem only lasted for 24 hours.

A. Diphenhydramine (DPH), the active ingredient in the antihistamine Benadryl, is very sedating. Studies have shown that it can impair the ability to drive safely (Annals of Allergy, Asthma and Immunology, March, 2004). It may also affect memory and concentration (Journal of Psychopharmacology, July, 2006).

Anticholinergic Drugs and Brain Fog:

A new study in JAMA Neurology (online, April 18, 2016) reports that taking anticholinergic drugs like DPH is “associated with increased brain atrophy and dysfunction and clinical decline.”

Such medications interfere with the action of an essential brain chemical called acetylcholine. There is growing concern that they could increase the risk for cognitive decline and dementia in older people.

There are dozens of drugs with anticholinergic activity, including most of the over-the-counter “PM” pain relievers. Some medications for allergy, depression, diarrhea, motion sickness, heart problems and overactive bladder may also affect the brain.

If you would like to see a list we have complied of anticholinergic drugs, here is a link.

Stories from Readers:

Naomi in Palm Beach Gardens, FL shares this story about a couple of anticholinergic drugs:

“I have taken Benadryl, Paxil and oxybutynin (maybe others?) FOR YEARS. What can I do to UNDO their effects? I have felt as though my memory is half gone for quite a while. I am 83 and made good use of a very good brain and a high I.Q. This information has been demoralizing and I hope you have some suggestions for reversing my mental direction.”

All three of the medications Naomi lists above (Benadryl for allergies, oxybutynin for bladder control and paroxetine for depression) have strong anticholinergic activity. We are not surprised that she has brain fog and memory issues after such a combined drug effect. There are alternate ways to deal with allergies (steroid nasal sprays or antihistamines without such strong anticholinergic activity, such as Allegra). Naomi should have a conversation with her physician to reduce the load of anticholinergic medications she is taking. Hopefully, that might undo some of the brain fog she is experiencing.

The original questioner might benefit from the same advice. This person appears to be especially susceptible to brain fog from DPH. We think such individuals must be extra cautious about other medications that have anticholinergic activity.

In our book, Top Screwups, we have a whole chapter devoted to drugs that can be especially dangerous for older people including a long list of medications that have anticholinergic activity. This person will want to check this list periodically and talk to his doctor to make sure any prescriptions he receives have a low risk for causing brain fog.