The Food and Drug Administration said Tuesday that manufacturers of testosterone drugs used by millions of Americans will be required to change labels for the drugs to warn that they could increase the risk of heart attacks and strokes and should not be prescribed to treat symptoms in men brought on by age, such as declining sexual drive.

In January 2014, the F.D.A. said it would reassess the safety of testosterone products based on two studies that suggested an increased risk of heart problems among men who used them. The agency will now require producers to conduct clinical trials to determine more definitively whether the products increase the risk of heart attacks and strokes.

A panel of outside experts advising the F.D.A. voted overwhelmingly in September that the labels should be changed to reflect the heart risks.

A spokeswoman for the agency said that companies have until March 11 to respond to the F.D.A.’s requirement and that the precise wording of the labeling will be worked out over time. She could not say exactly when the new wording would take effect.

Testosterone use has quadrupled since the early 2000s for men in their 40s, driven in part by a marketing campaign that suggests that the drugs are a solution for low energy, low libido and other ills, many of them associated with aging. In 2013, 2.3 million Americans were being treated with testosterone therapy, according to material distributed at the panel meeting last year.

Dr. Aaron Katz, chairman of the department of urology at Winthrop-University Hospital in Mineola, N.Y., said the evidence showing risks of the treatment was mixed, and he questioned the F.D.A.’s changes. But he welcomed the effort to consider more carefully how the therapy is used. He said that many patients who received it had not even had testosterone level readings, and that the new labels could serve as a speed bump in prescribing the therapy, though doctors could still prescribe it as they see fit, despite what the label says.

“This is more of a wake-up call for the industry and for physicians,” he said.

The F.D.A. has approved testosterone therapy for medical conditions including disorders in which the testicles do not produce testosterone because of genetic problems, or for damage from chemotherapy or infection. The drugs are not approved for conditions related to aging or declining sexual drive.

“We are requiring that the manufacturers of all approved prescription testosterone products change their labeling to clarify the approved uses of these medications,” the agency said in a statement.