Benadryl, anhydrosis, and One AC

Tl;dr: Benadryl can cause horses to stop sweating, so monitor them carefully.

I’ve learned something new. My mare has allergies, and after an unsuccessful response to Zyrtec a few weeks ago, I decided to try Benadryl (not on veterinary advice, just based on other people having success with it.) She was getting 15 25 mg tablets twice a day. I had a big bottle of the generic, and it ran out last weekend.

During the time she was on it, she was a little sleepy as expected. But I noticed, a few days before we ran out, that after a 5 mile ride (“long” for this horse), she was really tired, and her skin was very warm and she had not sweated, except under her tack. Oh no, I thought… Anhydrosis! Our former trail buddy, a Friesian gelding, was recently diagnosed and has to be kept in his stall with fans on, and can’t be ridden unless the temperature is below 70 degrees, so I was thinking DOOM!

I read up and decided to order some One AC, because it works for some horses and it’s inexpensive. She was started on it Thursday, 4 or 5 days after stopping benadryl, and on today’s ride, in high 70s, humid weather, she was not only more energetic (she wanted to canter… and go fast, not half-heartedly trot), she sweated! And her skin did not get warm at all.

So… was it the Benadryl causing the problem, and would she have sweated regardless of whether she got One AC or not, or is One AC a miracle supplement for her?

I’ve heard of horses that stopped sweating while on Tri Hist. The one I knew personally resumed sweating after the Tri Hist was done; no other action. So, I’d suspect the Benadryl too in your shoes.

I had a horse that stopped sweating when on hydroxyzine and benadryl (separate occasions). The allergy specialist vet that prescribed the hydrox swore it could not be the cause, but then I found others here on COTH reporting the same thing. What I remember, and this was years ago, is that my horse was fine after a period of time off the antihistamines…I want to say maybe a week, maybe more? I did not use One AC or any of the other remedies (a bottle of Guinness a day is one IIRC). That horse has passed on, but I’m really careful about using antihistamines on any of my other horses now, and watch them carefully.

In humans, there is research indicating that anticholinergic drugs can inhibit sweating. This does include Benadryl and hydroxyzine. I’m not aware of equine studies.

Claritin, Allegra, and Zyrtec have a different mechanism. I consulted my horse’s personal pharmacist on this matter (aka my husband, a human pharmacist who did a veterinary rotation at VA Tech and kept all his books) and his advice was “you probably shouldn’t give your horse Benadryl, if he develops tolerance to Zyrtec again try Claritin.” My vet said that was a sound recommendation.


One AC is pretty much worthless. I bought a yearling who’s TB mother also suffered from anhydrosis.
I tried EVERYTHING without success including One AC. Horse loved his beer though.

" Considering the dearth of treatment options, lack of a cure, and strong suspicion that anhidrosis is genetic—the odds of anhidrosis are 22 times higher in horses with a family history of anhidrosis—Laura Patterson-Rosa, D.V.M., started searching for the genetic cause of anhidrosis to help eliminate it from the equine population, much like HYPP. To achieve this goal, Patterson-Rosa requires 200 samples submitted from owners of anhidrotic horses or offspring of anhidrotic horses by December 2016. For more information on anhidrosis and to submit samples, visit the Brooks Equine Genetics Lab website."

I’m planning to finish out the jar of One AC and then see what happens. I do know a few people who have had good results with it, though.

(Also, just because something has a genetic component doesn’t mean it can’t be treated. I’d be dead if that was the case!)

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I’ve had two stop sweating on me. One from hydroxyzine and the other for no reason. One AC helped both of them. The One AC worked better when I added a little salt in with their feed. I forget how much salt, just enough to get them drinking a bit more (they are a small pony and a mini so it wasn’t much). The one that stopped sweating from the hydroxyzine was the harder one to “re-set” and get sweating again, but he did come around and has been fine since.

As per my vet, I used Benadryl last spring when my vet was out, to combat an allergic reaction to the flies. I only gave 8 25mg tablets twice a day. (1000lb mare) My mare had no side effects from the medicine. But each horse can react to medicines so differently, I am glad to see your mare did much better on the One Ac