Genentech has begun offering secret rebates to eye doctors as an apparent inducement to get them to use more of the company’s expensive drug Lucentis rather than a less costly alternative.

Under the program, which started on Oct. 1, medical practices can earn up to tens of thousands of dollars in rebates each quarter if they use a lot of Lucentis and if their usage increases from the previous quarter, according to a confidential document outlining the program that was obtained by The New York Times.

Lucentis, approved in 2006, is mainly used to treat age-related macular degeneration, the leading cause of blindness in the elderly. It costs about $2,000 for each injection, with an injection into the eye needed as often as once a month.

The competition comes from Avastin, another Genentech drug that has the same mode of action. While Avastin is approved only to treat cancer, many retina specialists say it works just as well in the eye as Lucentis. So they are using it off-label because each injection costs only $20 to $50.