Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Momenta Pharmaceuticals (NASDAQ:MNTA) shares jumped by more than 10% earlier today following news that the FDA had approved Novartis' (NYSE:NVS) and Momenta's biosimilar to Teva Pharmaceuticals (NYSE:TEVA) Copaxone.

So What: Copaxone is the best-selling drug used to treat relapsing multiple sclerosis. Last year, Teva Pharmaceuticals reported that sales of Copaxone totaled $4.2 billion, representing 20% of its global revenue.

The approval of Novartis' and Momenta's biosimilar to Copaxone, which will be sold under the brand name Glatopa, marks the second FDA approval of a biosimilar this year, and the first approval of a biosimilar to Copaxone.

How much of a dent Novartis and Momenta can make to Copaxone's market share remains a big question mark given that Teva Pharmaceuticals has been actively switching patients to a longer-lasting formulation of Copaxone that requires up to 50% fewer doses per week. As of December, Teva Pharmaceuticals had successfully transitioned 60% of its Copaxone patients to its new version.

Now What:  According to Momenta, Novartis is still determining the timing of a potential launch of Glatopa. That's likely because of a recent Supreme Court ruling that effectively reinstates a September 2015 patent expiration for Copaxone.

Regardless of the timing, its likely that this drug will be able to capture a significant amount of share from the original Copaxone formulation. In countries overseas that have been quicker to approve biosimilars, biosimilars have typically won between 30% and 40% market share.

If Glatopa can achieve a similar market share, then it could make Momenta an increasingly attractive stock to own. According to its licensing deal for Glatopa, Momenta and Novartis will split the profit on the drug. Additionally, Glatopa's approval triggers a $10 million milestone payment to Momenta, and Momenta can also earn additional milestone payments if Glatopa achieves certain sales milestones. For this reason, I think Momenta is a stock to watch during the coming year.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.