Any sudden change in behavaior should be look at as a possible medical symptom. It is highly unusual to have a dogs normal activity level change, Given that a known side effect of benadryl is sedation/lethergy it is not illogical to assume a conection.
Did the vet give you a reason for the Benydril, some prescribe it specifically sedative effect. As an antihistimine for controling allergy symptoms in dogs it is generally not very effective, Dogs immune systems do not responded to allergies like humans do so histamine play general a small roll. Any anti-histamine is only likely to be effective 15% of the time. Using all 4 classes of antihistamines 1 at a time for a minimium of 2 weeks on can increase the odd of finding one that does work up to ~30%
Human Drugs
Diphenhydramine (Benadryl®)
Diphenhydramine = Benadryl
Hydroxyzine = Atarax
Clemastine fumarate = Tavist
Chlorpheniramine = Chlor-Trimeton
Taken as a whole this could all be related to allergies, Most general practicing vets are ill equipt to deal with unravelling secondary and primarly cause of dermalogical condition and instead just focus on symptom relief rather than actual control of the problem You should seriously consider a refferal to a vet that specializes in dermatology in the end it can save a lot of money.
Did the vet give you a reason for the Benydril, some prescribe it specifically sedative effect. As an antihistimine for controling allergy symptoms in dogs it is generally not very effective, Dogs immune systems do not responded to allergies like humans do so histamine play general a small roll. Any anti-histamine is only likely to be effective 15% of the time. Using all 4 classes of antihistamines 1 at a time for a minimium of 2 weeks on can increase the odd of finding one that does work up to ~30%
Human Drugs
Diphenhydramine (Benadryl®)
50 mg twice daily for a 40lb dog is well withing the normal dosing. You should consult with the vet on of the other common antihismines used in dogs might be just effective with fewer side effects. However side effects of each include sedation/lethergy
Diphenhydramine = Benadryl
Hydroxyzine = Atarax
Clemastine fumarate = Tavist
Chlorpheniramine = Chlor-Trimeton