In 1999, $2.6bn worth of Prozac was sold
The 20-milligram tablets of fluoxetine, produced by Barr Laboratories, will cost about one-third less than Prozac.
After a five-year legal battle, a court in Indianapolis ruled last week that the patent for Prozac, which was owned by Eli Lilly, had expired. Eli Lilly sold $2.6bn worth of Prozac in 1999.
Prozac is estimated to have been taken by at least 38 million people since it came on the market in 1986 in Belgium. It is now sold in more than 100 countries.
Popularity
The drug ranks alongside the painkiller Aspirin and the erectile dysfunction treatment Viagra in a rare category where the brand name has become a generic term in itself.
The Barr chairman, Bruce Downey, said the launch of the generic drug was "the most significant in the history of our industry".
He said it would help bring down costs across the entire health care system, which has been criticised for bloated drug prices.
"Millions of American consumers will immediately begin to benefit from savings, and millions more who might have otherwise had to forego this medicine because of its high cost will have access to a more affordable version of Prozac," he said.
Prozac has generated more than $1bn annually for Eli Lilly, and the pharmaceutical giant fought to get its patent extended until 2003.
But on Thursday competition authorities at the Federal Drug Administration gave Barr permission to sell fluoxetine, the generic name for Prozac.
Prozac, launched in the United States in 1988, works by encouraging serotonin - a brain-messenger chemical - to remain in spaces around nerve endings instead of moving into the nerve endings themselves, a process which apparently reduces depression.
The low risk of side-effects and zero dependence rate have contributed to Prozac's popularity.