Sexplosion (1990)

Couple watching television, who then decide sex is much more fulfilling than the alternative (and it is, really). After seven minutes of pure sex , we fade in to Paul asleep, and Rrose still ready for more. So, she injects her lover with a mysterious aphrodisiac called “Sexplosion” – only to uncover one of those nasty side effects, he turns into a raging homicidal maniac with a deformed monstrous penis, who promptly chases her into the bathroom, does very nasty things to her, and the rest of the short is pure adulterated hell.