Genentech won federal approval Friday for a new drug to treat advanced breast cancer. But the company said it was experiencing manufacturing problems that could lead to a shortage of the drug.

The drug, called Perjeta, is meant to be used with Genentech’s existing drug Herceptin to treat breast cancers characterized by high levels of a protein called Her2, which amount to about 20 percent of total breast cancers.

The two drugs together are more effective than Herceptin alone, but at considerable extra cost.

Perjeta will have a wholesale cost of $5,900 a month for the typical woman, Genentech said. When used with Herceptin, which costs $4,500 a month, a typical 18-month course of treatment would be more than $187,000.

The company said it has programs to help people obtain the medicines if they cannot otherwise afford them.