Fusilev, or leucovorin, treats the side effects of a chemotherapy drug called methotrexate.
Spectrum expects $10 million to $15 million in Fusilev sales the first quarter and $80 million to $90 million in sales for the year. That's a drop of at least 33 percent from 2012 and is much less than analysts expected.
Total revenue is seen in the range of $160 million to $180 million for 2013. Analysts expected $290.8 million.
The company reported $267.7 million in total revenue in 2012, including $204.3 million in Fusilev sales.
In after-hours trading Spectrum shares fell $4.82, or 38.8 percent, to $7.61. That put the stock on pace to open Wednesday at its lowest price since October 2011.
The Food and Drug Administration had been looking for ways to increase the supply of leucovorin. In October it approved a version of the drug made by Sagent Pharmaceuticals Inc. In 2011 the agency allowed Teva Pharmaceutical Industries Ltd. to start importing the drug from Hungary.
In February Spectrum Chairman and CEO Rajesh Shrotriya said "plenty" of the drug was now available and he expected Fusilev sales to grow in 2013.
The Henderson, Nev., company thinks it will still turn a profit for the year despite the lower sales of Fusilev with sales of its non-Hodgkin's lymphoma drug Zevalin and peripheral T-cell lymphoma drug Folotyn improving from 2012.