In May, I suggested that investors might want to hold off on investing in Spectrum Pharmaceuticals (Nasdaq: SPPI), but it's looking more investable as time goes on.

Spectrum has the same problem that's plagued Momenta Pharmaceuticals (Nasdaq: MNTA): It's hard to value the unknown. In Momenta's case, it's the fear that it'll gain additional competition for its generic version of Sanofi's (NYSE: SNY) Lovenox. For Spectrum, it's the fear that generic competition will come back.

Fusilev, which goes by the chemical name levoleucovorin, competes directly with the generic-but-still-works leucovorin. Fortunately for Spectrum, generic-drug makers -- Teva Pharmaceutical (Nasdaq: TEVA) and others -- have had a hard time making luecovorin, which has allowed Fusilev to thrive. Sales of the drug used to avoid chemotherapy side effects skyrocketed from $6 million in the third quarter of last year to $41 million this year.

Valuing Spectrum is still difficult, though. How do you guess when Teva and others will get their act together? And then when -- dare I say, if -- they do, how do you estimate how many doctors will head back to the generic? Unlike a generic that's picked up at a pharmacy by a patient, cancer patients aren't likely to question their doctor about the cost of their medication, so we're basically wondering which doctors would prefer what they were using prior to the shortages versus what they're using now.

Every quarter that goes by pads the company's coffers and makes it a little easier to hold onto doctors that become more comfortable using Fusilev. Even with the company bumping up against its 52-week high set back in July, there would seem to be some room to run higher, although investors are right to remain a little skittish about the generic competition.

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