MATH PUZZLE: Can you replace...

The dentist pulls out a Novocain needle to give the man a shot, so he can extract the man's tooth. '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 to 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 tablet.'
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.