The Food and Drug Administration (FDA) announced on May 5 that it will restrict the use of the Johnson & Johnson COVID-19 vaccine to adults who can’t receive mRNA vaccines, because of the risk of potentially life-threatening side effects with the J&J shot.

The agency said the COVID-19 vaccine can now only be administered to people aged 18 years of age and older who can’t receive either the Moderna or Pfizer vaccines, both of which are two-dose vaccines and use mRNA technology. J&J’s single-shot vaccine uses adenovirus technology.

People 18 and older who want to receive the J&J vaccine “because they would otherwise not receive a COVID-19 vaccine” can also get it, according to the FDA’s statement. The agency didn’t elaborate.

Over the past several months, the FDA and other federal agencies have increasingly recommended that Americans get either the Moderna or Pfizer vaccines over the J&J vaccine. In December, for example, the Centers for Disease Control and Prevention (CDC) issued a recommendation that prioritized the Moderna and Pfizer vaccines over the J&J vaccine because of safety issues.