Chilling last words Bill Cosby's accuser heard before she was allegedly sexually assaulted.

The last words that Bill Cosby’s accuser heard before drifting into semi-consciousness were that the pills he had given her would help her relax, a Pennsylvania prosecutor said at the start of the comedian’s trial.

But the pills left the woman, Andrea Constand, unable to resist the alleged sexual assault as she lay on a couch in his suburban Philadelphia home in 2004, Assistant District Attorney Kristen Feden told jurors.

“Trust, betrayal and the inability to consent – that’s what this case is about,” Feden said, as Cosby listened a few feet away, brows furrowed.

“These three friends will help you relax,” were the words Cosby allegedly told Constand.

Dozens of women have accused the 79-year-old entertainer of sexual assault, but Constand’s case against the man she considered a mentor is the only one that was recent enough for criminal prosecution.

Defence attorney Brian McMonagle countered that Cosby is the victim of false accusations. He said that Constand’s initial allegations were investigated in 2005 and found to be insufficient at that time.

"Today I get a chance, with your help, to right a wrong," he told jurors. "I get a chance, with your help, to protect a man from the destruction of the rest of his life."

The allegations have effectively ended Cosby's long career, built on a family-friendly style of comedy that had earned him the moniker "America's dad." Cosby has denied any wrongdoing and said the encounter with Constand, who worked at Temple University, his alma mater, was consensual.

McMonagle took aim at Constand's credibility, focusing on inconsistencies in her accounts to law enforcement officials in 2005. Constand, who did not report the alleged assault for nearly a year, initially told police she had not maintained contact with Cosby after the incident. In fact, McMonagle said, she called him 53 times.

Prosecutors said they will call a psychologist to testify that victims of sexual trauma sometimes engage in seemingly irrational behaviour, including maintaining a relationship with their assailants.

Listen: In Australian, Luke Lazarus was acquitted of raping a woman in 2013, highlighting the blurry lines of consent in the court (post continues after audio...)

Another accuser whom Feden identified as Kelly Johnson will also testify during the trial. Johnson, who like Constand sought out Cosby's career advice, was also drugged and sexually assaulted in a "strikingly similar" manner in 1996, Feden said. McMonagle dismissed her account, noting she only came forward in 2015.

Prosecutors will use Cosby's own words against him, including a deposition taken during Constand's civil lawsuit in which he said he gave her Benadryl and admitted he had given the sedative Quaalude to young women with whom he wanted to have sex.

It was the unsealing of that deposition in 2015 that prompted prosecutors to reopen the Constand criminal investigation, more than a decade after it was first closed.