Double Dose

John goes to his doctor and asks for a double dose of Viagra. The doctor says, "I can't give you a double dose".

"Why not?" asks John.
"Because it's not safe", replies the doctor.
"But I need it really bad", says John.
"Why?" Asks the doctor.
"Because", says John, "My girlfriend is coming to see me on Friday; my ex-wife will be here on Saturday; and my wife is coming home on Sunday. Can't you see? I must have a double dose."
On hearing this, the doctor agrees but says, "OK, I'll give it to you, but you must see me on Monday so I can check whether you've suffered any side effects."
On Monday, John drags himself in. His arm is in a sling. The doctor asks, "What happened to you?"

John replies, "No one showed up."