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 J ohn.

'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'.