WASHINGTON – In a closely watched decision, the Food and Drug Administration on Friday approved an application by German drug-maker Sandoz and Momenta Pharmaceuticals of Cambridge, Mass, to make the first generic version of the widely used blood thinner Lovenox.

The approval positions Momenta and Sandoz to offer a cheaper but still lucrative alternative to Lovenox, which had sales of $4.5 billion in 2009, making it the 15th best selling drug in the world.

In its decision, the FDA also rejected an argument by Lovenox’s maker, Sanofi-Avenis, that its drug, which is made from sugar molecules found in heparin, a substance derived from pig intestines, is too complex to be copied with precision by makers of generic versions of the medication.

The Sandoz/Momenta team had been in a bitter, five-year battle with Amphastar Pharmaceuticals, of Rancho Cucamonga, Calif., to win the FDA’s nod to make the drug, which goes by the generic name enoxaparin.

Amphastar last year accused Janet Woodcock, the FDA’s top drug official, of a conflict of interest involving her past associations with Momenta scientists. The complaint was dismissed by the Department of Health and Human Services.