Seth Rogen Slams Justin Bieber, Calls Him 'Obnoxious' and 'Ungrateful'

March 13, 2014 — -- Funnyman Seth Rogen thinks Justin Bieber is a joke.

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.

Read: Justin Bieber Arrested for Alleged DUI

"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!"

Related: Seth Rogen Opens Up About His Mother-In-Law's Struggle With Alzheimer's

But he did say "Oh yeah, lots of times," when asked about Paul Rudd.