Can I give my 90lb retriever 100mg of benadryl once a day and 1 zyrtec 8hrs later?

I would call my Vet,before I gave him/her anything..

I agree, your vet would be the one to ask that question.
We do not know your dogs medical history.

What is he taking Benedryl for?
Are you out of Benedryl?

100 mg of Benadryl? That's 4 times the recommended dose for a 200lb human.

No

NO

NO

Don't give a dog any meds without a Doctor's order. That means a Veterinarian.

I wouldn't give my dog benadryl period. It makes the drowsey.. Plus I always thought with human medications, dogs received half.

That's a lot of medicine.. Humans wouldn't even take that much. Or require it for that matter.