The 31-year-old was recently alleged to be suffering from a serious heroin addiction by the National Enquirer.
However, the actor's representatives quickly dismissed the report which claimed he was on a $6,000-a-month habit.
His father Kit and stepmother Jeanette Krylowski have now alleged that Culkin is "heavily into drugs".
"Kit has seen the photo. But he can't talk about it. He just can't, because it hurts him too much. Of course he's worried, he's incredibly concerned," Jeanette told The Sun.
"When I saw the photos I thought, 'Oh no, this is not good. Either he's dying of some disease or he's heavily into drugs'."
The Home Alone star blocked his parents from controlling his fortune as a teenager, which eventually led to the breakdown of his relationship with his father.
Jeanette added: "Now we're hearing he is denying the allegations - but it certainly looks like he is on drugs. He looks awful.
"There will always be a sadness in Kit that I can't fix. Our house is always open to Kit's children. Kit still loves him and all of his family.
"I think if he ever got a letter from Mack or any of his kids, saying, 'Gee Dad, I'd really like to see you' or something, then he'd probably get back to them."