The newly approved drug, called brexanolone, will be marketed under the name Zulresso. It will cost, on an average, $34,000 per patient for a course of treatment, before discounts, according to the manufacturer, Sage Therapeutics in Cambridge, Mass.

Tiffany Farchione, acting director of the psychiatry products division at FDA, said in a statement that the medication represented “an important new treatment option” for a potentially life-threatening condition. The drug, she said, is administered intravenously for 60 continuous hours. The approval requires that it be administered under strict safety conditions because of concerns it can cause “excessive sedation and sudden loss of consciousness.”

The drug will be available to patients only through a restricted distribution program at certified facilities — such as doctors’ offices or clinics — where health-care providers can carefully monitor the patient. It will carry a “boxed warning,” which is the strongest warning required by the FDA.

The FDA-approved label says data from a study in which a dozen women who were breast-feeding received the drug showed that the medication is transferred to breast milk in nursing mothers. However, the amount that is passed to the infant is low, and available data “do not suggest a significant risk of adverse reactions to breastfed infants from exposure to Zulresso.”

The FDA said the drug’s efficacy was shown in two clinical studies in which participants received either the medication or a placebo and were then followed for four weeks.

Experts called the drug a major advance for a serious illness that does not get enough attention. “We don’t have any treatments that are anywhere near this effective,” said Jess Fiedorowicz, a psychiatrist at the University of Iowa and a member of an FDA advisory panel that recommended agency approval of the drug. “So this is groundbreaking in that regard.”

Women diagnosed with postpartum depression are currently treated with antidepressants and psychotherapy, but the drugs take four to eight weeks to be fully effective and generally have only a small to moderate impact. The new drug, by contrast, takes effect quickly and lasts at least 30 days, according to clinical studies.

Still, Fiedorowicz said, the cost and method of administration could prevent women from getting it.

Samantha Meltzer-Brody, a psychiatrist at the University of North Carolina at Chapel Hill, who led the clinical trials for the drug, said the medication is such an improvement over current therapies that she doubts the IV administration will discourage its use. She noted that postpartum depression, one of the most common complications of childbirth, is “underdiagnosed and neglected,” and that suicide is a major cause of maternal death.

“For women suffering, you can say, ‘You can come in and be treated and in 2.5 days it can go away, and not come back,’ " she said. In clinical trials, she added, the IV administration did not prevent women from getting the drug.

The main component of drug is allopregnanolone, “a breakdown product” of the hormone progesterone that affects the GABA neurotransmitters, which have a role in mood regulation, Meltzer-Brody said. She added that the exact mechanism of action is unknown.

Sage is developing another drug to treat postpartum depression and major depressive disorder that would be administered as a once-daily pill. The medication recently showed good results in a Phase 3 clinical trial. If approved, it could be a blockbuster, some industry analysts say.

The FDA’s advisory committee recommended approval of brexanolone in early November, but the agency delayed the green light to evaluate concerns about the small number of women who lost consciousness while receiving the drug. The agency’s safety requirements are designed to deal with those safety concerns.

Anna Glezer, a psychiatrist at the University of California at San Francisco, said she was encouraged to see an entirely new approach to medication for depression and, in particular, a product aimed specifically at women’s mental health.

She said mild to moderate cases can be missed because the fatigue and sleeplessness that accompany the arrival of a new baby are also some of the physical symptoms of depression. In her clinical practice, she said, she asks women: “Can you sleep when your baby is sleeping? Is your energy lower given how much sleep you’re getting?”

Postpartum depression also is frequently accompanied by anxiety, she said.

Given the way the drug will be administered, Glezer said she expects it to be used mainly on the most severe cases, especially women whose postpartum depression requires hospitalization.

The FDA action is the second important approval involving depression this year. Earlier this month, in biggest advance for depression in years, the FDA approved esketamine, also called Spravato, for people with major depression who have not responded to other treatments.