Actor Charlie Sheen on Tuesday (Nov 17) confirmed rumours that he is HIV positive.

Sheen, 50, told NBC's Today show that he was diagnosed about four years ago.

"I am here to admit I am HIV positive," he said. "It's a hard three letters to absorb."

Sheen said he does not know how he contracted the virus.

"It started... based on this series of cluster headaches and insane migraines and sweating the bed, completely drenched two three nights in a row.. I was emergency hospitalised," he told host Matt Lauer.

"I thought I had a brain tumour. I thought it was over."

Asked if he has confided in anyone about his condition, he said: "I've told enough that I trusted."

He confirmed earlier reports that his ex-wives Brooke Mueller and Denise Richards knew about his diagnosis. He also revealed that he told his oldest daughter, Cassandra, 30, about his condition. "It hit her hard but she recovered and she's tough like her dad. And she said 'wow'," he added.

He said he decided to go public with his condition partly because a prostitute who came over to his house took a photo of his anti-retroviral medications and threatened to sell the picture to tabloid magazines.

"Are you still paying some of these people?" asked Lauer. Sheen replied: "Not after today I'm not."

Lauer then asked: "So you think that by speaking this truth you get out from under that?"

"That's my goal. That's not my only goal. We'll talk more about that later," the actor said, adding: "I think I released myself from this prison today."

Sheen added that it was "impossible" that he had transmitted the virus to anyone even though he admitted to having unprotected sex with two people after he was diagnosed with the condition.

"The two people that I did that with were under the care of my doctor and were completely warned ahead of time," he said.

He added that he was upfront about his infection with all his sexual partners.

Sheen, who is three times divorced, played the womanising bachelor Charlie Harper on top-rated US comedy series Two and A Half Men for eight years before being fired in 2011 for bad behaviour that included cocaine-fuelled partying with porn stars and a conviction for assaulting his ex-wife.

Sheen appeared on the Today show with his doctor Robert Huizenga who said the actor has received strong anti-viral treatment which has suppressed the virus "to the point that he is absolutely healthy from that vantage".

The doctor also dismissed media reports speculating that Sheen has Aids. He said: "Charlie has none of those. He is healthy. He does not have Aids."

He added: "My biggest concern with Charlie as a patient is substance abuse and depression from the disease more than what the HIV virus could do in terms of shortening his life, because it's not going to."

Sheen said he has stopped doing drugs but admitted that he "still drinks a little".