The dentist pulls out a numbing needle to give the man a shot.

'No way! No needles. I hate needles' the patient said.
The dentist starts to hook up the nitrous oxide and the man objects.
"I can't do the gas thing.. The thought of having the gas mask on is suffocating me!"
The dentist then asks the patient if he has any objection to taking a pill.
"No objection," the patient says. "'I'm fine with pills."
The dentist then returns and says, "Here's a Viagra."
The patient says, "Wow! I didn't know Viagra worked as a pain killer!"
"It doesn't" said the dentist, "but it's going to give you something to hold on to when I pull your tooth."

H/T Old Dude