The Food and Drug Administration has warned the manufacturer of a long-acting anti-addiction medication that its printed advertisement does not include one of the most serious risks of using the product: the increased chance of an opioid overdose.

In a Dec. 2 letter to Alkermes that was posted to the FDA’s website Wednesday, the agency said a company ad for Vivitrol does not warn users that they are more vulnerable to a “potentially fatal” overdose if they resume use of opioids after the medication wears off.

“The print ad is false or misleading because it omits important risk information associated with the use of Vivitrol,” the FDA wrote.

Vivitrol is an extended-release version of naltrexone, which blocks the effects of prescription and street opioids, preventing relapse. It is administered by injection every 28 days, after drug users eliminate opioids from their systems. While on Vivitrol, a user’s tolerance to opioids declines.

As Vivitrol’s effect wanes, or if users miss a dose or stop using it altogether, they risk overdosing if they take a legal or illegal opioid in the same amount they were previously consuming, the FDA said in a news release. Caregivers often provide similar warnings to former users who get off opioids in rehab or while incarcerated.

“This violation is concerning from a public health perspective because it creates a misleading impression regarding the overall safety of Vivitrol,” the agency told Alkermes in its letter. The company’s product labeling includes the warning, it said, but the printed ad that appeared in a journal does not.

A major weapon in the public health battle against the opioid epidemic, which kills about 130 people each day, is increased medically assisted treatment with medications such as Vivitrol, buprenorphine and methadone, accompanied by counseling and other measures. In 2018, Alkermes reported $83.8 million in net sales of Vivitrol, up 11 percent from 2017.

In a statement, Alkermes said the warning pertained to an ad in a journal that is “not currently in use.” It said it was finalizing a response to the FDA due by Monday and is “committed to working closely with the FDA to ensure that all of our promotional materials are fully compliant with applicable laws and regulations.”

A spokesman for the company declined to comment further.

The FDA told Alkermes to develop a plan to distribute “truthful, non-misleading and complete corrective messages” to anyone who received the incomplete ads.