The US Food and Drug Administration has opened the door to the first stream of generic formulations of Eli Lilly's antidepressant Cymbalta (duloxetine), following the recent loss of its patent protection.

The agency said it has approved various strengths of the drug on offer from Aurobindo Pharma, Dr. Reddy’s Laboratories, Lupin, Sun Pharma, Teva Pharmaceuticals USA and Torrent Pharmaceuticals.

Duloxetine and other antidepressants carry a boxed warning describing the increased risk of suicidal thinking and behaviour during initial treatment in children, adolescents, and young adults ages 18 to 24, it noted, stressing that the drug must be dispensed with a patient medication guide outlining crucial information on its uses and risks.

So, what does this mean for Lilly? Well, Cymbalta is a popular antidepressant which, with annual sales just shy of $5 billion a year, is not only the firm's top-earner but also the world's fifth biggest seller. So the arrival of cheaper copycat forms will no doubt take a huge chunk out of its sales.

In fact, if the sales curve follows the usual post patent loss trajectory, Lilly could be left reeling from a 70%-80% hole in Cymbalta sales in just a year, having had to weather a similar landslide after its antipsychotic Zyprexa (olanzapine) lost its patent armour in 2011.

The drugmaker made a swathe of salesforce cuts over the summer to help it better cope with the fall of its drug, but also ramped up its diabetes team to prepare for the looming launch of two late-stage therapies in development, which it is hoping will help claw back some sales losses.