Lester Cohen / WireImage.com 1 / 5
Lester Cohen / WireImage.com 1 / 5

Robert Downey, Jr. has spoken out to reassure fans after Zach Galifianakis sparked up a joint on U.S. TV, insisting his "Due Date" co-star was smoking "fake weed."

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"The Hangover" funnyman stunned viewers when he lit up during an appearance on Real Time With Bill Maher on Friday night, puffing away as he spoke about California's upcoming vote for marijuana legalization.

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But Downey, Jr., who appears onscreen with Galifianakis in the new road trip comedy, admits his pal wasn't smoking real drugs in the TV studio.

He tells Entertainment Tonight, "His answer to that is that it was real fake weed."

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