Sexual addiction has suddenly become a fashionable topic for films. The ground was broken by Steve McQueen’s 2011 “Shame,” which presented an unflinching portrait of a man caught up in the condition, told in a sleek, arty and quite explicit way. Now Stuart Blumberg goes the earnest, hopeful route in “Thanks for Sharing,” a dramedy that means well but feels forced and unconvincing at virtually every turn. And that despite a capable cast.

The script centers on three guys in a support group. The senior member is Mike (Tim Robbins), the owner of a small construction firm, a genial but intense fellow who’s always on call for those who need his help in resisting their urges. He’s sponsor to Adam (Mark Ruffalo), a hard-driving environmental consultant who’s celebrating a long run of sobriety. The newcomer is Neil (Gad), a young emergency-room doctor who’s lost his job for trying to add sneakily-filmed up-the-skirt workplace shots to the large collection of porno he keeps at home. Adam agrees to be his sponsor, but it quickly becomes apparent that Neil’s in the group only because of a court order, and initially he’s not serious about taking the mandated steps.

Each man goes through an obligatory crisis as the plot proceeds. Mike is confronted by the return of his estranged son Danny (Patrick Fugit), who comes home claiming to have overcome his addiction to drugs on his own (something that his father seriously doubts)—a situation that causes friction with his supportive wife Katie (Joely Richardson). Meanwhile Adam, encouraged by Mike to start dating again, strikes up a romance with intelligent, beautiful, athletic Phoebe (Gwyneth Paltrow), but fails to reveal his condition to her despite knowing that she’s shell-shocked as the result of a previous relationship to an addict and doesn’t want a similar experience. And Neil, who’s still under the thumb of his mother (Carol Kane), appears headed for disaster until he intervenes to help another group member, ebullient hairdresser Dede (Alecia Moore), and for the first time experiences romantic feelings with another real person.

“Thanks for Sharing” veers wildly between intense dramatic sequences and broadly comic ones, most of the latter involving Neil’s efforts to get around New York City without using the subway—a place rife with temptation. (His solution, of course, is to take up bicycling, and the sight of a chubby guy desperately trying to maneuver a bike around crowded streets is supposed to be a scream.) And there are sequences that, whatever the intent, come across as positively weird—like the one in which Phoebe, who’s supposed to be a fairly sensitive person, decides to perform a virtual strip-tease routine in front of Adam to turn him on. (You might also roll your eyes over the suggestion that her devotion to physical fitness is a sort of addiction too, even if you’re the positively sedentary type.)

Under the circumstances the cast do all they can to sell the material, but they’re all far from their best. Robbins outshines the others, largely because Mike has the greatest number of obstacles to address—his domestic affairs, his support of Adam, his attempt to help another member of the group by giving him a job—and some of his scenes with Fugit and Richardson have a soap operatic effectiveness. Ruffalo employs his capacity to scrunch up his face in an expression of intense suffering overmuch—it sometimes looks as though he’s just experiencing a stomachache—and the last-act sequence in which he takes up again with a needy former one-night stand turns into pure melodrama. Gad’s efforts at comic relief reek more of desperation than of real humor, but he does manage to add a touch of pathos to Neil along the way. Pop star Moore, aka “Pink,” does not impress in her first film role, and Kane’s drippy shtick has outlived its usefulness. On the technical side, the picture is just as mediocre as it is on the narrative one, with nondescript cinematography by Yaron Orbach.

“Thanks for Sharing” is clearly may be heartfelt and well-intentioned, but it comes across like an adult version of an afterschool special.