a fella goes to see his dentist with a sore tooth, the dentist offers him some gas for the pain.
"nope .............. gas will make my throat swell up, i'm allergic to it" he says.
next the dentist offers an injection for the pain, "nah ............ i'll break out in hives everywhere i'm allergic to that as well"
at this point the dentist gets a couple of little blue tablets out of the drawer and offers them to the bloke.
"no, i'm sorry mate i can't take any pain killers whatsoever" the fella says.
"i know that mate, they're viagra ......... at least they'll give ya something to hang onto when i pull that tooth out"