A once-a-month injection to treat alcoholism won federal approval Thursday, expanding availability of a drug previously sold only in daily pill form.

The Food and Drug Administration approved Vivitrol, spokeswoman Laura Alvey said. Cambridge, Mass.-based Alkermes Inc. will make the injectable form of the drug, also known as naltrexone. Cephalon Inc., of Frazer, Pa., will market and sell it.

The companies hope monthly injections of the drug, to be administered at a doctor’s office, will prove an easier regimen for alcoholics to follow than the daily pill. The drug is to be used in conjunction with counseling or group therapy, the companies said.

The drug works by blocking neurotransmitters in the brain believed to be associated with alcohol dependence, diminishing the craving for alcohol.