That's pure evil, but for best effects combine the viagra with the ex-lax... they'll then have to decide which is the most embarassing, walking to the toilet looking like they've pitched a tent in their boxers, or just staying in the car and enjoying the warm squishy feeling!!!
I got viagra'd the once by a mate, didn't leave the house much that night...
One of the best tricks I pulled involved a video camera and one of my really shy female friends...
I had a party round at my house, and before everyone arrived I stood in the loo for about 10 minutes videoing the empty toilet. So at the party, everyone got nicely drunk, and the girl in question decided to go to the loo. So while she was out the room, I hooked the camera up to the telly, gathered everyone around the TV and explained that when she came into the lounge again we were all to p!ss ourselves laughing, and start playing the tape of the loo I recorded earlier... she comes back, we kill ourselves, she looks at the TV to see what we're laughing at and on screen she sees my toilet so she thinks we've all just watched her go for a slash!
She went lots of funny colours, but she did eventualy see the funy side... about three weeks later!!!