"Selling Sunset" star Nicole Young is pushing back on serious claims levied by Chrishell Stause and Amanza Smith ... telling TMZ she's never had a substance abuse problem.

We got Nicole at Royal Smoothie Studio City in L.A. and our photog asked her straight up if she's ever had a drug problem, as Chrishell and Amanza claimed during this season's "Friendsgiving" episode.

Nicole tells us she's never had a problem with substances and says Chrishell and Amanza know that full well ... which she says makes their public claims all the more "heinous."

The Friendsgiving fight between Nicole and Chrishell was explosive, and it resulted in Nicole being put in timeout ... she went MIA from the show and now she's telling us she felt like she was singled out. Netflix

Nicole says she's the

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