Which raises a question: What the fuck? More specifically, where the hell did he get that stuff, and why did he have it with him?
For those unaware, Thorazine is an antipsychotic, also known as a major tranquilizer, usually used for treating psychosis or obsessive-compulsive disorder. It also has a nasty side effect of impairing judgement, causing sleepiness, and making it hard for the body to move. Before going to the apartment, Peter had no reason to believe Dana would be possessed. The possibility had never even been raised. He also knows that he can’t use this drug on ghosts. There’s only one person he could have planned to use it on, and it sure as hell wasn’t Rick Moranis’ character. (Not yet, anyway.)
While we’re at it, we’re pretty sure Venkman got that stuff illegally. Neither parapsychology nor psychology are medical fields, so the only likely way a guy like Venkman could have obtained it would be through bribing a corrupt pharmacologist. Also, that amount of Thorazine is waaay above the lethal dosage. If Dana hadn’t been possessed by a ghost, she would have ended up as one herself. Of course, the alternative is that Peter carries enough date rape drugs for 12 women on his person at all times, just to be efficient.
To his credit, Peter does reject Zuul’s offer to get down with Dana’s body (after thinking about it), so at least there’s that. But the question is: Was the Thorazine bit a plot convenience they rightfully knew most viewers wouldn’t think too hard about (“Eh, they probably had to use the stuff in a previous possession case”), or was it a subtle joke they tossed into the script, that Dr. Venkman’s the type of guy who brings knockout drugs on his dates? It’s not like the film up to that point wasn’t full of jokes demonstrating that Venkman thinks about sexual consent the way corporations think about tax laws.
Either way, as with lots of these “hilariously shitty guy” types, Venkman gets the girl in the end. He proves his worth to Dana by killing the demigod and saving the world, freeing her from the possession — which, oh by the way, he refused to take seriously the first time they met (he spends the whole time hitting on her). And to be clear, Venkman doesn’t change at all, or learn anything. All that happens is that Dana learns that Venkman is in fact as awesome as he claimed to be. His dickish, horndog exterior is something she as a woman has to get past if she wants to land a man. Or, you know, get a male professional to spend time on the issue she called about.