can i give my dog more than one benadryl a day for his itchiness and rash that is caused by his flea collar until the morning?

This was posted almost 24 hours ago, but I'll answer anyway in case someone needs this:

For Benadryl, the general rule of thumb is 1 MG per LB of weight, every 8 hours max. One example is if dog is stung by a bee, you need to gently remove the stinger (so it doesn't continue injecting toxins) and you'd give the appropriate amount according to dog weight up to 3 times in 24 hours, and if the dog was still uncomfortable seek veterinary attention.

If your rat terrier weighs about 8 lbs, and your Benadryl is 25 MG caplet, you'd only want to give a third of a tablet at a time.

Deb is correct. It goes by your dog's weight as to how much you can give him. My dog is 15 lbs and the vet said a half a Bendadryl was okay. But he might be twice your dog's weight.

Also you may want to look into using something other than a flea collar if it means your dog has to be on Benadryl. There are so many other options.

I have to agree with Karen on looking for options other than a flea collar, although they do make collars that use natural ingredients to repel insects and I've used them on my own dogs. I've known many animals that have had adverse reactions to them as well as one dog that got terminally ill after her owner put one on her for the first time. I think of chemical treatment as a last resort whenever possible. I know this wasn't your question so I hope you don't mind my opinion.