There are two halves to “50/50” that wouldn’t be very entertaining if they each expanded into its own full-length movie.

A movie about fighting a rare form of cancer isn’t many people’s idea of escapist entertainment. And watching Seth Rogen laugh through yet another raunchy stoner comedy wouldn’t be so annoying had he not already made three or four movies exactly like it.

Combining the two, however, turns out to be a winning formula in “50/50,” a very funny story about cancer that contains moments of powerful, devastating drama. The pot jokes help to ease the blow.

Joseph Gordon-Levitt (“500 Days of Summer”) stars as Adam, a healthy young man who is diagnosed with a rare form of spinal cancer. His girlfriend (Bryce Dallas Howard) and his mother (Angelica Huston) freak out after the news, but best bud Kyle (Seth Rogen) stands strong with some alternative therapy options – notably getting high and chasing women.

Rogen is responsible for much of the humor in “50/50,” proving that a little bit of his brand of rambling pop culture references can go a long way in a supporting role. The obnoxious laughing that drowns most of his previous films is limited to a few instances, and in one scene, Gordon-Levitt hilariously mocks the Rogen chuckle.

Gordon-Levitt is right up there with Ryan Gosling as one of our best young performers, and he delivers another resonant performance as Adam, a guy who isn’t accustomed to relying on others for support. Some of the strongest moments take place during Adam’s chemotherapy, where he connects with other people at various stages of the disease.

“50/50” especially shines in the scenes between Adam and a young, in-training counselor, played with warmth by Anna Kendrick. Like her performance in the George Clooney drama, “Up in the Air,” Kendrick channels the frail eagerness of a smart person trying to compensate for a lack of real-world experience. Gordon-Levitt and Kendrick have strong chemistry, but the movie smartly doesn’t cut corners in making them an immediate romantic item.

The film is inspired by screenwriter Will Reiser’s experiences facing cancer at a young age, and “50/50” above all plays like a genuine experience with few Hollywood-ized plot turns. The friendship between Adam and Kyle especially rings true, probably because Rogen was a close friend of Reiser during his cancer bout.

“50/50” is the kind of smart comedy that often gets forgotten during awards season in favor of more heavy-hitting drama. Blame the stoner humor, I guess, but without it, “50/50” wouldn’t be able to pull off its grim subject matter. It’s a strange but surprisingly insightful combination.

Grade: A-