Found on my iGoogle page, with elaborations:

So this 56 year old guy with alcoholic cirrhosis calls me up to complain about the Viagra he picked up in Mexico.

I’m all like, “Sir, you really shouldn’t be taking Viagra.” I mean, his systolic blood pressure is like 85 mmHg at baseline.

“Well, I’m stopping the lactulose,” he tells me.

“Why?”

“I can’t tell whether I’m coming or going.”