HOUSTON (CW39/AP) – U.S. health officials are warning against the overuse of the only drug available to treat monkeypox, saying that even a small mutation in the virus could render the pills ineffective.

Dr. Anthony Fauci, the nation’s top infectious disease official, noted that resistance is always a risk when using antiviral drugs.

“That’s why we’re uncomfortable when you only have a single drug,” Fauci told reporters.

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

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, including drugs, regulators stressed that doctors should be “judicious” in prescribing the medication.

Meanwhile, the Centers for Disease Control and Prevention 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 the Centers for Disease Control and Prevention, documenting their patient’s need and agreeing to track their results and any side effects.

The update came as federal officials on Thursday expressed cautious optimism about the trajectory of the outbreak, noting that new cases have fallen about 50% since their peak in August.

The Harris County Public Health Department reported 28 cases of monkeypox for Sep 5, 2022 to Sep 12, 2022. One month ago, Harris Count reported 140 cases between August 5, 2022 to August 12, 2022.*

During a White House briefing, CDC Director Dr. Rochelle Walensky attributed the decline to vaccinations, educational outreach and individuals reducing behaviors linked to spread.

Local health officials from Houston and Harris County want anywho is thinking about getting the monkeypox vaccine to know they have enough doses readily available. They encourage those looking for the vaccine to reach out and schedule an appointment.

If you would like to schedule an appointment for your monkeypox vaccine, you can call the Harris County Public Health hotline at 832-927-0707. The Houston Health Department’s hotline is 832-393-4220.

To read more about monkeypox, you can visit the monkeypox guidance page at ReadyHarris.org or visit HoustonHealth.org.

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* SOURCE: Harris County Public Health Department, https://publichealth.harriscountytx.gov/monkeypox

The Associated Press contributed to this report.