Start with benadryl as mattyg said - 1mg/lb - most benadrly comes in 25mg tablets.
If benadryl doesn't work there are some pretty good allergy meds available...we used Apoquel for a while..made a big difference. Our labs allergies have settled down quite a bit so we're back to just giving benadryl when needed...which here lately it's been a lot - just that time of year.
Apoquel worked great for our dog that needed to clear up some skin allergies prior to getting surgery one time. Benadryl works great, and as the others have said, you probably werenít giving your dog enough. Itís a very safe drug for humans and dogs.