Does long term use of antihistamines cause dementia?

DEAR DOCTOR K:

I’ve been taking over-the-counter antihistamines for years to control my allergies. Now I hear I may have to worry about dementia. How real is the concern?

DEAR READER:

Antihistamine drugs have “anticholinergic” (an-tee-cole-in-ER-jik) effects. That means that they have some tendency to block the action of a natural substance called acetylcholine. This substance transmits messages in the nervous system. In the brain, it is involved in learning and memory; in the rest of the body, it stimulates muscles to contract.

Drugs with anticholinergic effects include tricyclic antidepressants, antihistamines, some muscle relaxants, some intestinal relaxants, treatments for dizziness and drugs that treat overactive bladder. Between about 10 and 40 percent of older adults take drugs with anticholinergic effects. We’ve known for a long time that these drugs can cause temporary symptoms such as sleepiness, confusion, and problems with short-term memory and reasoning.

Earlier this year, researchers published a study in the journal JAMA Internal Medicine which indicated that these drugs also may increase the risk of a long-term problem: dementia. The research team studied nearly 3,500 men and women ages 65 and older. They noted all the drugs, both prescription and over-the-counter (OTC), that each person took the 10 years before starting the study. Then the team tracked the participants’ health for an average of seven years.

During that time, 800 of the volunteers developed dementia. The researchers found that people who used drugs with prominent anticholinergic effects were more likely to have developed dementia than those who didn’t use them, and also that the risk of dementia increased with the cumulative dose. The people who took the highest doses for the longest time had a 54 percent higher dementia risk than people who took the lowest doses for the shortest time.

Several prior studies have come to the same conclusion as this study. These prior studies had included only prescription drugs. This study is the first to include OTC drugs. In that way, it is more complete.

This study and the several before it that come to similar conclusions are disturbing. Drugs with anticholinergic properties are widely used; I regularly take a medicine (fortunately in low dose) that has anticholinergic effects. I think this study was particularly well done.

The body produces less acetylcholine as we get older, and blocking its effects can deliver a double whammy to older people. So what does one do when presented with evidence like this? I think we have to balance the benefits we know we are getting from the drug against the possible risks. We also have to ask ourselves if we have any options — other drugs that treat the same symptoms, but that do not have prominent anticholinergic effects.

If you regularly take an anticholinergic drug, talk to your doctor about alternatives. For example, selective serotonin re-uptake inhibitors (SSRIs) are a good alternative to tricyclic drugs if you have depression. Newer antihistamines such as loratadine (Claritin) can replace first-generation antihistamines. And Botox injections and bladder training can help to improve incontinence.