Viagra
Male Patient: "Doctor, can you prescribe me three Viagra tablets, please?"
Doctor: "Er, well, actually I'd rather not...it's not really a good idea to prescribe, well, so many in one go."
Male Patient: "Let me explain...on Friday, my girlfriend is staying over; on Saturday my ex-wife is staying over and on Sunday my wife is returning from holiday. I know it's very unusual, but this is a one-off - I assure you."
Doctor: "I'm still not sure about this...Right, I'll give you the tablets on one condition - you come and see me first thing on Monday morning so that I can carry out a check-up, okay?"
The patient readily agrees, takes the tablets and goes merrily on his way.
On Monday morning, the doctor is in his office. In comes the patient, with his arm in a sling.
Doctor: "What on earth happened to you?"
Patient: "Nobody turned up."