American Beer

This guy goes to a doctor and says he has a problem with sex.
"Doc, I think my dick is just too damn small," he says.
The doctor asks him which drink he prefers.
Well, American beer," he replies quite bemused.
"Aaaahhh. There's your problem, it shrinks things, those silly American beers.. you should try drinking Guinness.
That makes things grow."
Two months later the chap returns to the doctor with a big smile on his face.
He shakes the doctor by the hand and thanks him.
"I take it you now drink Guinness?" asked the doctor.
"Oh no, Doc," replies the man, "but I've got the wife on American beer!"