First “Dexter,” now “Nurse Jackie.”

Showtime is in danger of becoming a network that clings to its onetime hits for several seasons longer than it should.

Not only is “Nurse Jackie” back for a sixth season that takes the story of drug addict ER nurse Jackie Peyton (Edie Falco) back to where she started in season one — she’s popping pills like they’re M&Ms again — but Showtime already renewed the series for a seventh season.

Too bad. Running a show into the ground only hurts its legacy. Look no further than “Dexter,” which ran too long and got saddled with a terribly dull, unsatisfying ending, for proof.

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As season six begins (9 p.m. Sunday, April 13), Jackie is still with her cop boyfriend, Frank (Adam Ferrara), but unbeknown to him, she’s back on drugs even while attending AA meetings.

Jackie is also estranged from her daughters in ways that don’t mesh with the character as established in earlier seasons. Even when Jackie was on drugs previously, she always stayed on top of what was happening with her kids; it stretches credulity that she’s surprised to learn her oldest daughter is in an AP biology class.

Another head-scratcher: Jackie scams her way into getting an OxyContin prescription but doesn’t seem to know that in New York there’s a 24-hour waiting period before she can pick it up. Really? Isn’t that the sort of thing any experienced drug addict, especially one who is a nurse, would know?

At one time, it was possible to enjoy “Nurse Jackie” even for viewers who were over Jackie’s drama because the supporting cast was so much fun. But there’s something less entertaining about them these days, too. Their plots sometimes feel like a stretch, as if the writers are grasping for stories for them.

The cat’s out of the bag about Zoey (Merritt Wever) dating Prentiss (Morris Chestnut). And Dr. Roman (Betty Gilpin) is back and once again teasing Coop (Peter Facinelli), who dreads turning 40.

In the new season’s first two episodes, the show’s writers introduce a new friend for Jackie, an understandable development after the departure of Dr. O’Hara (Eve Best) last season.

Like O’Hara, this new friend, Antoinette (Julie White), is outspoken with a self-proclaimed “big personality.” But the writers try to change up the dynamics of the friendship by making Antoinette part of Jackie’s AA group, allowing Jackie to be more honest with this new character than she can be with her work colleagues. She comes clean quicker to Antoinette than she did to O’Hara. It’s a welcome effort to try a twist on what worked in the past, but at this point in the show’s life there’s not much that hasn’t been done.