The National Institutes of Health (NIH) said on Friday it had started enrolling monkeypox patients in a late-stage study testing Siga Technologies Inc's antiviral pill Tpoxx against the disease.

The oral and intravenous formulations of Tpoxx are approved by the U.S. Food and Drug Administration for the treatment of smallpox, but does not yet have clearance to treat monkeypox.

It is, however, currently accessible by clinicians for treating monkeypox under a compassionate use request.

The NIH aims to enroll more than 500 patients, including both adults and children, who will then be randomized to receive either Tpoxx or placebo pills for 14 days.

Investigators will evaluate if participants receiving Tpoxx heal more quickly compared to placebo, as well as provide critical data on the optimal dosing and safety of the drug in children and people who are pregnant.

The United States has recorded more than 21,000 confirmed cases of monkeypox, according to data from the Centers for Disease Control and Prevention.