to be treated

Hello all,

pls help, which is correct:

1) He was treated with aspyrin.
2) He was treated by aspyrin.

I think that the second sentence could be correct, because
I suppose that you can be treated by a doctor with a medicament. but I am not sure as I have found also sentences with "with" on the internet.

By the way, does anybody know a good online English dictionary specialized at prepositions? Prepositions are a source of my greatest trouble:-(

Thanks

Natalia