Funnyman Seth Rogen thinks Justin Bieber is a joke.
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In the midst of Bieber's legal drama, which included an alleged DUI back in January, Rogen, 31, tweeted taht he's not a fan of the singer.
Then, last night on "Watch What Happens Live," he was asked to explain the harsh tweet.
"In my opinion, Justin Bieber is a piece of s--t," he told Andy Cohen. "He's obnoxious and ungrateful and insincere."
He took things to a serious level when he said Bieber, 20, "puts people's lives in danger" -- likely referring to the singer's DUI arrest.
The actor also dished on a list of Hollywood stoners, which sadly doesn't include best bud James Franco, whom Rogen said "doesn't smoke weed!"
But he did say "Oh yeah, lots of times," when asked about Paul Rudd.