Can I give my 110 lb. Lab a Benedryl tablet daily for scratching?

I would rather say consult your vet first. Although benedryl is used for cats and dogs who are not on any other medication, it may inerteract unfavourably or fatally if with other meds. Benedryl liquid has too much alcohol in it to be safe for dogs, so use a capsule or pill form. Here is the chart of dosage:

Dosage every 8 hours
Dogs under 30 lbs and Cats : 10 mg
Dogs 30-50 lbs: 25 mg
Dogs over 50 lbs: 50 mg

It's always best to discuss giving human medications with your vet before hand. We don't know your dogs heath history. Assuming he is suffering from allergies and no other health issues, Benedryl is one of the more commonly used anti histamines for dogs. You would probably give him two pills twice a day. The normal dosage would be 50 mg for dogs over 50 lbs.

You should contact your vet for a check up. Just because a dog itches, doesn't automatically mean its allergies. Talk to your vet and find out your options. They can also give you proper dosage per lb for medications.

Never just give your dog people medicine because you "heard" it works. Every case is different.. Every body is different. Your dog may not even have allergies and you'd be wasting your money on purchasing it.

Good luck

You should find out the issue first, but it may help until you can get him to the vet.
Assuming your dog is in good health, the "norm" is 50mg for dogs over 50lb. But, the actual dose is 2-4mg/kg. Start low, so with 50mg. You could go up to 100-200mg however. You can give it every 8 hours (three times a day MAX)
Then, call your vet Monday.