Scott Baio Regrets Erin Moran Drug Comments, Didn't Know She Died from Cancer

Scott Baio feels terrible for ripping his co-star, Erin Moran, for her drug use and says he never would have done it if he'd known she was battling cancer.

Scott tells TMZ he was wrong to insinuate Erin -- his "Happy Days" and "Joanie Loves Chachi" co-star -- had died from drug-related problems. He went pretty hard on her during a radio interview Monday morning.

He tells us he went to bed Sunday reading reports she'd possibly died of a heroin overdose, and went into the interview with that in mind. He was upset he'd lost Erin to drugs after she wrestled with substance abuse for years.

However, hours after he went off the air ... officials in Indiana announced Erin had likely died from complications of stage 4 cancer. Scott says if he had known, he would've kept his mouth shut.

He tells us Erin never disclosed her diagnosis to him -- even after his wife, Renee, spoke to her just last month. He adds ... Renee has battled cancer herself several times, so losing Erin this way hits close to home.