Sunburn
A poor chap falls asleep whilst sunbathing.
When he wakes, he finds his legs all red and burned, very painful.
He goes to the doc, who looks at his legs, shakes his head and reaches into his cabinet.
"Here you go" he says handing the man some tablets
"Take one of these every night for the next few days"
The man strides off but just as he gets to the door he notices the tablets are blue.
"Ere!" he says, "these look like viagra tablets"
"They are" says the Doc
"Well will they cure my sunburn?" asks the man
"No!" says the Doc
"But they will keep the sheets off your legs whilst you're sleeping !"