Alkermes PLC has received a warning letter from the U.S. Food and Drug Administration regarding its print advertisements for opioid dependence treatment Vivitrol, which did not disclose the most serious risks associated with taking the drug.

The FDA posted its warning letter Dec. 11, stating that Alkermes' advertisements for Vivitrol included the injection drug's benefits, but not the risks of vulnerability to opioid overdose. The FDA also said the Vivitrol advertisements omitted information about other common adverse events, such as injection site reactions.

"The print ad is false or misleading because it omits important risk information associated with the use of Vivitrol," the FDA wrote in the letter. "Thus, the print ad misbrands Vivitrol within the meaning of the Federal Food, Drug and Cosmetic Act."

Alkermes is expected to respond to the letter by Dec. 16, according to the FDA's press release. The regulator requested that the Dublin-based company immediately halt its Vivitrol advertisements containing misleading information, and come up with a plan to disseminate accurate information about Vivitrol to consumers who may have received the previous advertisements.

Alkermes' advertising violations are "serious," the FDA said.

Vivitrol is an extended-release injection formulation of naltrexone, a commonly used substance to prevent relapse of opioid dependence. Vivitrol works by blocking the effects of opioid drugs, which can result in a higher response rate to a lower dose of opioids. If a patient takes opioids after taking Vivitrol, an overdose could occur, the FDA said.

"This violation is concerning from a public health perspective because it creates a misleading impression regarding the overall safety of Vivitrol," the agency wrote in the letter.

The opioid crisis is estimated to kill about 130 people per day in the U.S.