Diphenhydramine Side Effects for Dogs (Benadryl)

So, I gave Belle her first in a long time dose of 25 mg of diphenhydramine last night with food. This was around 7:30 p.m. We sat down to have dinner and then watch some TV. Belle slept like a rock. A couple of times I tried to wake her up by calling her name, which usually does the trick. However, she just kept on sleeping. Now, she could have been worn out from the vet’s visit; she did get her 6-month exam and shots at the same time, but it didn’t really seem like it.

This morning, she was sluggish to get up and moving and just seemed a little loopy. We just checked on her again and she seems to be getting better.  However, this prompted me to start looking in to the side effects of the allergy medicine on dogs.

The first site I went to was written by a veterinarian and discussed the side effects of diphenhydramine or Benadryl. The common ones they listed were

  • sedation
  • dry mouth
  • retaining urine

Ok, that explains the sleepiness. The vet also went on to mention that the side effects might be stronger in older dogs and that different dogs may react differently and the dosage should be tailored to your animal. Belle definitely reacts poorly to most medicines, as seen in her urinary incontinence side effect with both prednisone and antibiotics. Even though the vet recommended 25 mg dose twice a day, I think we will try out once a day for a while. If Belle’s sluggishness doesn’t go away, but she is still itching, we will cut that dose in half and try that.
Read more about the side effects and details about diphenhydramine.


  1. says

    I’ve had some allergy bouts with my beagle mix, Hendrix. It always seems like that first dose of diphenhydramine makes her VERY sleepy, but she seems to acclimate to the doses after about 24-48 hours and return to her usual self. For me, Benadryl might as well be a sleep aid…knocks me right out!

  2. BUD says


  3. Katelan says

    Bud, this is happening to my dog as well. She’s 50 lbs and we’ve been giving her Benadryl daily off and on for about 6 months.

    She will get the exact symptoms you’ve described sometimes before bed. If you find anything out, I’d love to know.

  4. Carlyn says

    I have a 90 lb golden retriever and she has the same symptoms of running around like she’s on speed, then she will stop and stand in one spot for an extended period of time anywhere from 10 min to 30 minutes staring at the wall, then it seems like something startles her and shes off again running around. I’ve wondered if she’s hallucinating because her behaviour is so unusual. Any thoughts?

  5. Rose says

    It is the 4th of July and we were going to go to a neighbors house. My dog hates fireworks so I gave her benadryl before we left. Just came home and her left leg is swollen from her foot to her hip and she can’t stand on it. Other than that she seems normal. I’m going to take her to a vet in the am. I’m just worried and that is how I found this website.

  6. Belle's Mom says

    I’m afraid I don’t know the answer to this. When you find out from your vet, please post here to help others.

  7. bmdd says

    my vet gave my appx 9 year old 90 lb lab/spaniel mix (JACK) a 50 mg dose of diphenhydramine (not for any allergies but before performing a “fine needle aspirate.”) Jack was so gassy (much more than usual) and he had diarrhea. Is this normal for this to happen?

  8. says

    I’m sorry to hear about your poor dog. I am not qualified to answer this question as I am not a vet. I did some research on PetMD and Vetinfo.com and they both list diarrhea as a possible (but rare) side effect of Benadryl (which is diphenhydramine). I would definitely discuss this with your vet. This article, written by a vet, provides more details on side effects, precautions, and drug interactions: http://www.wedgewoodpetrx.com/learning-center/professional-monographs/diphenhydramine-for-veterinary-use.html I hope this helps!

  9. Sherrill says

    Carlyn;Hello we have a 5year old sheltie that started exhibiting very similar behaviour except for the running around,in his case it is more like severe restlessness.The vet has diagnosed it as seizures caused by anaplasmosis,a tick borne infection that affects the Liver functions. Regards Sherrill

  10. Wilkys says

    We gave our 3yo terrier Tinkerbell benadryl for her allegeries and she has consistantly mounted our other dog. She seems calm and just quieter now.

  11. tara says

    I give my 25lb Boston Terrier 1 25mg benadryl tablet daily for itching…he tolerates it well and it helps him get a little grogy so I give it at bedtime or close to it… we’ve had no repercussions … he takes trifexis monthly for fleas and I don’t give him the benadryl that same day…

  12. Mindy says

    I’ve been giving my 50lb beagle mix 50mg of Benadryl twice a day for about 2-3 weeks, and I feel like his itching has been worse lately. Does anyone know if he could be allergic to the Benadryl?

  13. Hiranya says

    Our Puppy had a severe case of swollen eye which the ER vet assumed to be allergy related. She prescribed 12.5mg benadryl tab for her (18lbs). She became absolutely loopy and seemed to have lost all motor coordination, Then she threw up several times, all through the night. She behave d as if she was under the effect of anesthesia. HAs anybody seen it?

  14. kaylon says

    To the ones talking about their dogs acting like they’ve taken a hit of speed, diphenhydramine affects some children like that as well. I wouldn’t be too concerned with it.

  15. Jason says


    We are going through the same thing now. Our dog is acting like he doesn’t know us. He is a completely different dog. Did your pup go back to normal and how long did it take?

  16. says

    My 68.7 pound Beagle was having an Allergic Reaction to Benedrly he could not stop itching his Paws stopped the Benedrly and the itching has stopped

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