My niece called me and said her 3month old terrier mix ate one of her alegra pills. Is there any real concerns for this? Will he show side effects? What to watch for

Why wait for us to answer when you can get this info from the vet almost immediately. The dog could already be suffering by now ! Yes, I think it could be critical, wouldn't you wonder if that would be the case !

So how long now has it been since the dog ate the meds??

How many and how long ago?
Is it he having any signs of side effects or unusual symptoms?
It sounds like you should call a vet.