The jury in Bill Cosby's sexual assault trial have begun their deliberations over whether the veteran entertainer drugged and molested a woman more than a decade ago.

A conviction could send Cosby, 79, to prison for the rest of his life.

Closing arguments gave differing portrayals of what happened between Cosby and Andrea Constand at his suburban Philadelphia estate.

Defence lawyer Brian McMonagle said Cosby and Ms Constand were lovers who had enjoyed secret "romantic interludes", insisting the 2004 encounter was consensual.

McMonagle said that while the comedian had been unfaithful to his wife, he did not commit a crime.

Prosecutors said "fancy lawyering" could not save Cosby from his own words - namely, his admission about groping Ms Constand after giving her pills he knew could put her to sleep.

"Drugging somebody and putting them in a position where you can do what you want with them is not romantic. It's criminal," district attorney Kevin Steele said.

Cosby did not take the stand, ending days of suspense over whether the jury would hear directly from him.

Legal experts said testifying would have been a risky move that could have opened the TV star to withering cross-examination about some of the 60 or so other women who have accused him of drugging or molesting them.

He is charged with three counts of aggravated indecent assault, each one punishable by up to 10 years behind bars.