This’ll sound like some kind of comedian’s roast, but it’s an honest question. For a striving young screenwriter, which is the worse break: being Paul Reiser’s cousin or randomly getting cancer? Will Reiser is two for two there, and some people will be wondering, albeit maybe not aloud, whether he really deserves to have a movie made about his life.

It is after all a funny movie, and an honest one, and that goes a long way, or at least a little longer than an hour and a half. But the big question hovering over “50/50″ has to do with how it handles its privileges. Not dying is a privilege, of course, and Reiser’s script gets into that with winning (if also sometimes sitcomish) humility.

Reiser is said to have gotten the second of those aforementioned breaks while working on “Da Ali G Show” with his pal Seth Rogen, who then jokingly but also seriously suggested a comedy about a very reluctant young cancer patient and his obnoxiously goofy friend. Now here it is, with Joseph Gordon-Levitt as the Reiser proxy and Rogen as, basically, himself.

Gordon-Levitt isn’t exactly breaking new ground either, but he’s as easy as ever to watch. A stoic sufferer, cautious enough to wait for a walk signal before crossing an obviously empty street, he’s positioned very early as an apt straight man for the absurd disease, whose symptoms manifest mostly in nearby people’s reactions.

His girlfriend, played by Bryce Dallas Howard, vows to support him but can’t live up to her vow. His mother, played by Anjelica Huston, was overbearing even before the diagnosis. His psychotherapist, played by Anna Kendrick, is younger than him, and still quite awkwardly only a trainee (and inevitably, sitcomishly, also a love interest). His fellow chemotherapy patients, played by Philip Baker Hall and Matt Frewer, offer warm, vulgar, too-brief camaraderie. And his best buddy, ever loyal but at a loss, just wants to set him up with some medical marijuana and get him laid.

The director of “50/50” is Jonathan Levine, who debuted with “The Wackness,” a little slice of ’90s-slacker-wigga poseurdom, and can be said to have progressed if only for now trying less hard. Levine lets Reiser’s script breathe, lets the actors do what they’re good at, and seems only to intervene when nudging the proceedings along with obvious soundtrack songs.

The result is less an envelope pusher than a difference splitter, the common denominator between buddy comedy and affliction drama. Reportedly it once was called “I’m with Cancer,” which gives a better sense of the attitude, and of the essential relationship. The new title, a reference to our protagonist’s coin-flip odds of survival, also adds a telling whiff of equivocation. But at least the movie itself doesn’t turn into a lecture on how humor helps us cope.

Reiser’s real privilege is the vitality of youth. Better to get the story from him than from some pseudo-wise old coot whose boring memoir you have to patronize with pity laughs. And maybe in a decade or two, when cancer comedies have gotten very sophisticated, we’ll look back on “50/50” and congratulate ourselves, like survivors, on having come so far.