Yeah, this is one of those "Why does Criterion have THIS?" situations. I picked it up at the library on the sheer basis of the title and Criterion alone. It does show what a fine actor Karloff was, the atmospheric setting of the bar/brothel where much of the action took place was well-done, and it was totally cool to see such a young Christopher Lee in action. But essentially it's a "Drugs Are Bad, Mmkay, Even In the Name of Research" PSA.

I would say it is a classic for medical horror genre, and very rare case where the main villain in the movie is ... pain itself. Very refreshing.

This is a really good movie. Karloff is great in this film and the movie has a very interesting story. Personally really enjoy it but it is not a horror film, not even a thriller. It is just a drama and a very good one.