Amanda Bynes. Picture: Bang Showbiz

Actress Amanda Bynes admitted she is "really ashamed and embarrassed" about a period in her life when she a lot of drugs and posted a number of bizarre and hurtful messages about her family and other celebrities, including some infamously vulgar posts about Drake, and the comments still "eat away" at her.

She said: "I'm really ashamed and embarrassed with the things I said. I can't turn back time but if I could, I would. And I'm so sorry to whoever I hurt and whoever I lied about because it truly eats away at me. It makes me feel so horrible and sick to my stomach and sad.

"Everything I worked my whole life to achieve, I kind of ruined it all through Twitter. It's definitely not Twitter's fault - it's my own fault."

The 32-year-old actress even made the "really foolish" decision to announce her retirement on the microblogging site.

Speaking to Paper magazine for their Break the Internet issue, she said: "I saw it and I was convinced that I should never be on camera again and I officially retired on Twitter, which was, you know, also stupid.

"If I was going to retire [the right way], I should've done it in a press statement -- but I did it on Twitter. Real classy! 

"But, you know, I was high and I was like, 'You know what? I am so over this' so I just did it. But it was really foolish and I see that now. I was young and stupid."

Amanda - who has been sober for four years - admitted her substance abuse "messed up" her brain and turned her into a different person.

She said: "My advice to anyone who is struggling with substance abuse would be to be really careful because drugs can really take a hold of your life.

"Everybody is different, obviously, but for me, the mixture of marijuana and whatever other drugs and sometimes drinking really messed up my brain. It really made me a completely different person. I actually am a nice person. I would never feel, say or do any of the things that I did and said to the people I hurt on Twitter."

And the 'She's The Man' star warned even "harmless" drugs can ruin a person's life.

She continued: "There are gateway drugs and thankfully I never did heroin or meth or anything like that but certain things that you think are harmless, they may actually affect you in a more harmful way. Be really, really careful because you could lose it all and ruin your entire life like I did."