Millions of Americans consume Benadryl each year to alleviate allergy symptoms, but medical experts have discovered an alarming connection between this medication and a severe illness.

The active ingredient in Benadryl, diphenhydramine, is known for blocking histamine receptors. Histamine is a chemical released by the immune system that can cause symptoms such as a runny nose and itchy skin.

However, medical professionals warn that these affordable tablets can penetrate the blood-brain barrier and interfere with receptors for the neurotransmitter acetylcholine, which is essential for processes like attention, memory, and learning.

One study found that Benadryl’s active drug and similar medications raised the risk of dementia by 54 percent and Alzheimer’s by 63 percent among those who take the drug regularly compared to those who do not use the medication.

Other studies have linked the drug to a higher risk of drowsiness, falls and car accidents, with people in Europe advised not to drive after taking the substance.

‘There have been several studies over the past decade that show an association between Benadryl and dementia, particularly in older patients,’ Dr Joao Lopes, a pediatrician at Case Western University, told the Daily Mail.

‘It is not our first option and it has not been for a long while, given the existence of safer and more effective medications.’

Still, Dr Anna Wolfson, an allergist at Massachusetts General Hospital, said, ‘If you’re on a deserted island and Benadryl is the only allergy drug available, you should take it. But, in almost every other case, there are safer, better options.’

A spokeswoman for Kenvue, which makes Benadryl, said in a statement, ‘We are not aware of any studies that show a causal link between labeled use of diphenhydramine and an increased risk of developing dementia. Diphenhydramine is an ingredient which is generally recognized as safe and effective by health authorities in the US.

‘Safety is always our top priority, and we recommend that consumers carefully read and follow the label when using any medication.’

In a major study into Benadryl’s potential links with the cognitive disorder published in JAMA Internal Medicine, researchers tracked 3,400 people over 65 years old in Seattle, Washington, including 2,200 individuals (64 percent) who took antihistamines like diphenhydramine.

Patients were recruited in 1996 or in the years between 2000 and 2003, and were tracked for more than a decade. None had dementia at the start of the study.

Over the 10-year period, there were nearly 800 cases of dementia recorded (23 percent of all the participants).

Researchers found those taking at least two doses per week of an anticholinergic medication – like Benadryl or an antidepressant – had a 54 percent higher risk of dementia compared to those who were not taking the drug.

The study was observational, and could not definitively prove that there was a link between dementia and Benadryl, although it was part of a growing body of literature suggesting a possible connection between the two. 

Doctors said that taking the medication occasionally was safe, but warned those who take the drug more regularly or around two to three times per week should consider speaking to their healthcare provider.

In place of Benadryl, physicians recommended newer antihistamines like cetirizine, often sold under the brand name Zyrtec, or fexofenadine, sold widely as Allegra.  

Dr Wolfson, told The New York Times, ‘Occasional use [of Benadryl] is fine and nobody needs to run home and throw out their stash.’ 

Dr Olalekan Otulana, a physician in the UK, said, ‘The risk appears to be more significant when these types of drugs are used frequently and over many months or years, and particularly in older adults.

‘Occasional use (say for a short bout of hay fever or trouble sleeping) is unlikely to carry the same level of concern.’

He added that ‘People should not panic if they have taken it in the past. But they should have an informed conversation with their doctor if they are using it regularly.’ 

Benadryl has been available in the US since the 1940s, with many taking the drug simply because they did during childhood.

In some European countries, like the UK, Benadryl does not contain diphenhydramine, instead swapping it got newer and safer antihistamines like acrivastine or cetirizine.

In Germany, Sweden and some other European countries, access to first-generation antihistamines like diphenhydramine has been restricted. 

It was not clear why Benadryl maker Kenvue uses different ingredients in the US compared to elsewhere in the world.

About 1.5 million prescriptions for diphenhydramine are written in the US every year, with millions more given out over-the-counter.

Many doctors have previously called for people to use alternatives to Benadryl for years.

In a paper published in February this year in the World Allergy Organization Journal, doctors wrote, ‘Diphenhydramine, once a pioneering antihistamine, is now overshadowed by second-generation antihistamines with similar efficacy and fewer adverse effects.

‘Based on a comprehensive evaluation of practice patterns and the prevalence and incidence of adverse clinical events, we believe that diphenhydramine has reached the end of its life cycle, and in its class of therapies it is a relatively greater public health hazard. 

‘We recommend it should no longer be widely prescribed or continue to be readily available over the counter.’