The Food and Drug Administration has approved Chelsea Therapeutics’ (NASDAQ:CHTP) drug to treat a rare condition that can cause patients to black out due to low blood pressure.

But Chelsea’s drug Northera will need to carry a so-called “black box” label warning patients about the increased blood pressure while lying down, which the warning says can lead to cardiovascular events. The agency said that patients using Northera must sleep with their upper bodies elevated.

The FDA’s decision on Northera, Chelsea’s first approved drug, comes comes nearly two years after regulators declined to approve the compound. Besides safety concerns, the FDA was questioned whether the duration of the drug’s effect lasted longer than one week.

Chelsea licensed Northera, also called droxidopa, from Japanese drug company Sumitomo Pharma. Droxidopa has been used in Japan since 1989 and generates about $50 million in annual sales. Chelsea studied the drug as a treatment for neurogenic orthostatic hypotension or NOH, a rare condition for which there are no safe alternatives available in the United States. 

NOH is a chronic neurogenic disorder resulting from deficient release of the hormone norepinephrine. Patients who have NOH experience dizziness and fainting upon standing due to low blood pressure. Northera works by being converted in the body into norepinephrine, which increases blood pressure. Chelsea describes NOH as “feeling that you are about to black out.” The condition can be found in some patients with Parkinson’s disease, multiple systems atrophy (MSA) and pure autonomic failure.

NOH is a rare condition affecting between 80,000 and 150,000 people in the United States and Europe, Chelsea said. The FDA approved Northera under its accelerated approval program for drugs that treat rare diseases and address unmet medical needs.Northera’s label states that the drug’s effectiveness beyond two weeks has not been demonstrated. Northera has conditional approval as long as Chelsea conducts additional studies to confirm that it works. Chelsea said that it has reached preliminary agreement with the FDA on the parameters of this study, which would enroll approximately 1,400 patients over a six-year period.

Now that Northera has FDA approval, Chelsea CEO Joseph Oliveto said that the company expects to be able to deliver the drug to patients in the second half of this year. Chelsea last month hired on a chief commercial officer charged with leading the Northera commercialization work.

The therapy may generate $450 million in peak U.S. sales, Robyn Karnauskas, an analyst with Deutsche Bank AG, said Feb. 14 in a note to clients after a panel of agency advisers voted overwhelmingly to recommend Northera’s approval. FDA staff members had expressed concern that Chelsea hadn’t proved the drug works longer than a week while advisers pushed for flexibility because no other treatments exist for the condition.

“This approval is a significant step forward for people with MSA forced to cope with the debilitating effects of NOH,” Judy Biedenharn, co-president of the Multiple Systems Atrophy Coalition, said in a statement. “Addressing the major symptoms of NOH can have an impact on a person’s ability to perform even the simplest of daily activities — like taking a shower or standing to brush one’s teeth — offering an important benefit to both individuals and their caregivers.”

(Bloomberg News contributed to this report)