In “Pineapple Express,” Seth Rogen is Dale, a 25-year-old process server who enjoys weed, talk radio and the company of his high school girlfriend, in that order. While enjoying his first joint of the new, rare kind of grass referred to in the title, he witnesses his next defendant — and a policewoman — commit murder. He chucks his roach out the window and jets, crashing noisily into the cruiser — but the murderer is the drug kingpin who provides the supply for Dale’s dealer Saul (James Franco). Since the stuff hasn’t hit the street yet, there are only a few people who would have access to it — fingering him almost immediately as the witness.

The first 80 percent of “Pineapple Express” is a different breed of escape farce because it doesn’t take any of this seriously. Rogen comes off as spontaneously unscripted, the way “Napoleon Dynamite” did before they turned every other line into a T-shirt. When he sees the policewoman charge into the drug kingpin’s mansion, he quips, “what an adorable little cop.” After realizing, back at Saul’s, that there might be a link back to them, he vomits on Saul’s desk and says, “that was chicken fries.” “You threw up on my printer,” Saul says. “Did you break it?” Then there’s a bumbling fight that involves a Dirt Devil and a small potted cactus as weapons.

The genuine irreverence ends about the time of the cop car chase scene, which is pure “Bad Boys 2.” By the time we get to the final showdown, “Pineapple Express” has veered into a Casper Van Dien/Dolph Lundgren stunt orgy. There’s an ambush by an Asian drug gang. Rogen and Franco become unsmiling experts in weaponry. There’s actually a ventilation duct escape and a two-wave firestorm from a slow-mo-thrown suitcase bomb that can only be escaped by diving.

What happened, “Pineapple Express”? It’s like you went out to get Hot Fries and let Jon Turteltaub fill in the blanks with leftover notes from “3 Ninjas.” The irony of this movie is that you can pinpoint the exact moment in it where Rogen and his crew ran out of the mellow, paranoia-free, doublemint-scented chronic — and decided to finish the movie sober.