Cosby previously said under oath that he got several prescriptions for quaaludes in the 1970s to give to drug women before having sex with them.
Cosby has denied all of the women's claims, saying that any sexual encounters were consensual.
David Harris, a professor at the University of Pittsburgh School of Law who specialises in criminal justice, said the hung jury did not mean that a civil jury would clear Cosby of liability.
He said Andrea Constand, the woman at the center of the case (she's accusing Cosby of drugging and sexually assaulting her in his Philadelphia home in 2004), is also on board and willing to testify again. Prosecutors reopened a criminal probe and filed charges in 2015 after Cosby's deposition in the woman's 2005 lawsuit was unsealed, and they realized the 12-year statute of limitations for felony sexual assault had not yet expired.
The alleged assault that sparked the charges took place in early 2004 after Cosby invited Constand to his home in Elkins Park, just outside Philadelphia, to discuss her career plans.
Yet the judge had his limits, putting his foot down Friday when jurors asked to hear a sliver of testimony they'd just had read back to them. His attorneys have said throughout that their client is innocent of all accusations.
With deliberations passing the 50-hour mark, longer than the testimony of all the witnesses combined, the 79-year-old TV star's lawyer complained that jurors were seeking a replay of the entire trial.
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Cosby, who turns 80 next month, was subdued and seemed nearly dazed when O'Neill declared the mistrial.
A conviction could send Cosby to prison for the rest of his life, but the case has already helped demolish Cosby's nice-guy image, cultivated during his eight-year run as Dr. Cliff Huxtable on "The Cosby Show", the top-rated 1980s and '90s sitcom.
NBC10 does not typically identify people who say they are victims of sexual assault unless they grant permission, which Constand and Johnson have done.
Cosby's defense lawyers, however, argued that Constand's account could not be trusted after they pointed to numerous discrepancies in her statements to police in 2005, when she first reported the encounter, which occurred almost a year earlier.
Before the trial, Cosby told CNN's Michael Smerconish that he wants to return to comedy and activism.
"The key to retrying a case is to do it differently the second time, because the defense expects you to do it the same way", said Constand's lawyer, Dolores Troiani.
With no decision in sight, the TV star thanked his fans and supporters - first in a tweet, then in brief comments as he left the courthouse late Friday. The jury also awarded $8.5 million in compensatory damages to Goldman's family.
But only after a man, comedian Hannibal Buress, called Cosby a rapist in a stand-up act in October 2014 did "public outcry begin in earnest", Bowman wrote in a Washington Post op-ed, repeating allegations that Cosby drugged and raped her. If you would like to discuss another topic, look for a relevant article.