U.S. health officials are warning against overuse of the lone drug available to treat monkeypox, saying that even a small mutation in the virus could render the pills ineffective. 

The Food and Drug Administration (FDA) updated its guidance this week for Tpoxx, which has been prescribed to tens of thousands of patients with the virus. 

In an online update, FDA officials cautioned that a single molecular change to monkeypox "could have a large impact on the antiviral activity of Tpoxx." Since viruses are constantly evolving to overcome obstacles to infection, including drugs, regulators stressed that doctors should be "judicious" in prescribing the medication. 

Meanwhile, the Centers for Disease Control and Prevention (CDC) on Thursday said Tpoxx should no longer be given to otherwise healthy adults who are not suffering severe symptoms. 

"For most patients with healthy immune systems, supportive care and pain control may be enough," agency officials said in a statement.

The moves to scale back Tpoxx's use follow weeks of criticism from HIV advocates and other patient groups who have urged the Biden administration to make the antiviral drug more widely available. Tpoxx is approved for the related smallpox virus, and its use against monkeypox is considered experimental and tightly controlled by federal officials.

Doctors wishing to prescribe the drug must submit an application to CDC, documenting their patient's need and agreeing to track their results and any side effects. Officials have shipped 37,000 courses of the drug to physicians. 

Tpoxx works by targeting a single protein found on monkeypox, smallpox and similar viruses. The FDA said this week that research in labs, animals and people suggests multiple ways in which monkeypox could develop resistance to the therapy. 

The FDA approved the medication in 2018 under its "animal rule," which allows approval based on animal data when human testing is unethical or unfeasible. Smallpox was declared eradicated in 1980 by the World Health Organization, ruling out the possibility for human studies.

The CDC reported last week that 3.5 percent of patients tracked through its Tpoxx program reported side effects, mainly headache and nausea. 

The agency has only gotten back about 200 forms from physicians documenting patient's initial symptoms and results, accounting for less than 1 percent of the doses shipped since the start of the outbreak.

(With input from AP)