Kim Kardashian -- No Patience for Rob Kardashian's Issues with Drugs and Weight

Kim Kardashian is tired of her brother wallowing in his problems, and admits she's taking a tough love approach until he takes action.

Kim was on "Watch What Happens Live" Tuesday night and told Andy Cohen what went down between her and Rob when he left Italy before her wedding to Kanye West.

As TMZ first reported ... Rob ditched because he was self-conscious over his weight gain and did not want to appear in any wedding photos.

Last night Kim said, "I try to encourage him, and once you don't make that change for so long ... my patience wears out." She never says exactly WHAT he's been dealing with "for so long" -- but our Kardashian sources say Rob's weight gain is closely tied to his issues with drugs.