I know the OP asked about Portugal. But I did find this about Spain.
I am hoping one of two things may apply, that either you could hop across the border given both Portugal and Spain are Schengen countries. Or it may also mean Portugal has similar rules. But
External Link/Members Only"
Is it possible to buy Viagra without a prescription in Spain?Is Viagra available over the counter in Spain? Yes, indeed. In Spain you can find many chemist’ shops that offer over-the-counter medications, which usually require prescription, issued by a licensed specialist. Viagra OTC and Cialis are not the exception. For these drugs, prescription is not needed in Canaries, Spain and Balearics as well. However, we are not sure, whether it is legal or not."