Eli Lilly has filed a lawsuit against generic drugmaker Barr Pharmaceuticals in a bid to block it from selling a copycat version of its once-weekly formulation of the antidepressant Prozac.

The 90mg version of Prozac Weekly (fluoxetine) remains under patent in the USA, although other formulations of the drug are now open to generic competition. Sales of the product are currently running at around $40 million, according to Barr, which claims to be the first company to file for a rival version.

Barr’s application was filed in 2001, but the US Food and Drug Administration has still not approved it. Lilly claims it has a patent on the weekly formulation that extends out to 2017.

Prozac generated billions of dollars in its prime but Lilly’s exclusivity on the product was cut short after an earlier than expected loss of a principal patent in the USA.