I read this on the BBC News website this am. 

"Talisker, a border collie, was living on borrowed time after vets found his heart had become dangerously enlarged.

The dog, three, from Yardley Hastings, Northamptonshire, passes out when he gets excited because his blood cannot pump quickly enough around his body.

But, since taking Viagra, the blood flow to Talisker's heart has improved, according to owner Lesley Strong, 58.

Vets advised Ms Strong to give Viagra to the dog after they found he was suffering from a potentially lethal heart condition."

What a wonder drug for both man and dog!!