You'd think that two walking Petri dishes wouldn't have so many issues making out, but you'd be wrong.

Reports from the 'Scary Movie 5' set indicate Lindsay Lohan actually refused to kiss Charlie Sheen in the flick -- and for any scenes requiring mouth-to-mouth, a body double was used.

TMZ says Lohan was originally slated to smooch Sheen three times during their sex scene in the raunchy horror spoof, but she wouldn't do it because of his hard-partying, hooker-enthusiast past. (And probably present.)

What's more? Both parties had to sign releases saying neither of them had cold sores at the time of filming. Okay, ew. Ew. EW.

Apparently Lilo warned producers early on that she had real issues kissing the erstwhile cocaine enthusiast, and considering he's had more dental work than George Washington, we can sort of understand. As it turns out, Charlie was cool with it too, and had no problems with her (he later even gave her a rather lovely gift in her time of need).

Then again, prior to her bleaching and veneers, La Lohan wasn't a beam of dental hygiene either. Seriously.

Are we sure all this wasn't how the Toxic Avenger was spawned?