INVESTIGATED FOR PULLING KNIFE ON DENTIST

Charlie Sheen clearly doesn't like going to the dentist, because he's under investigation for assault with a deadly weapon for allegedly going nuts in a dental chair in a rock cocaine-fueled rage ... TMZ has learned.

Law enforcement sources tell us ... Charlie was at an L.A. dental office last Thursday for an abscess. He sat in the chair as the dental technician placed a mask over his face to administer nitrous oxide, when she says Charlie went insane ... flailing his arms and striking her.

The technician told cops she walked out of the room as Charlie's security guard entered. Charlie's personal dentist was there along with the oral surgeon. There was a lot of commotion and the technician says when Charlie's dentist finally came out he told her Charlie pulled a knife and went after him. The dentist wasn't hurt.