Bynes tweeted on Tuesday: "Ur ugly @mileycyrus." She included a photo of Cyrus at the recent Billboard Music Awards.
A week ago, Bynes also posted a little something about Miley's fiance: "Liam Helmsworth (sic) is the most gorgeous man on the face of the earth other than Tanz Watson. FYi!"
Some have wondered whether Bynes' latest tweet is a publicity stunt. Or perhaps she's jealous of the former "Hannah Montana" star who has successfully transitioned from being a Disney star to a big-screen actress and musician.
Others believe Bynes may be having a mental breakdown.
In response to the tweet, Cyrus told Hot 99.5 FM, "I think it's all so sad, and I don't want to add any fuel to that fire...I was rooting for her comeback until she started attacking me. I just thought it was hilarious that she mentioned my name the moment I was the worldwide trending topic on Twitter."
Meanwhile, Byne's former co-star Drake Bell claims there's nothing wrong with her. He recently told OK! magazine:
"Amanda and I are close... I talk to her every day. She's a sweetheart. I had lunch with her yesterday, and she's brilliant. She's good, and she's healthy... She's in a place right now where she is searching. She's ready to be her own person, and do her own thing... She is a girl who is finding herself."
Early this morning, Bynes tweeted: "For those asking, I'm voicing my opinion. I've been asked if I do drugs, I don't do drugs for the last time!"
She also wrote about her former crush Drake: "I won't marry @drake because I want to marry a gorgeous man! Sorry ur ugly @drake but I have dated ugly guys before so you stand a chance!!!"