A Cambridge biotech said Tuesday that patients who received its experimental drug for long COVID for four weeks showed a “statistically significant’’ lessening of fatigue, the most common symptom of the puzzling disorder that experts estimate afflicts 25 million Americans.

Axcella Therapeutics, a small biotech created by the venture capital firm Flagship Pioneering — which 12 years ago birthed the Cambridge-based coronavirus vaccine maker Moderna — said the drug was no better than a placebo at improving recovery from moderate exercise as measured by amounts of phosphocreatine, the study’s primary goal. It did, however, lead to an improvement in patient scores on a questionnaire measuring mental and physical fatigue.

Based on the tiny mid-stage clinical trial involving 41 patients, Axcella said it hopes to talk with US and British regulators soon about the fastest way to get the drug approved.

There is no approved treatment for long COVID, a debilitating condition that lingers after people no longer test positive for the coronavirus. The symptoms also sometimes include shortness of breath, heart palpitations, and brain fog. Long COVID can follow even mild cases of the illness and linger for weeks, months, or years, according to the US Centers for Disease Control and Prevention.

The clinical trial at the University of Oxford in England found that 21 patients who had been suffering from long COVID for at least three months reported that their mental and physical state improved significantly after taking the drug in a powdered drink twice a day, compared with 20 patients who received a placebo. Recipients of the drug also walked further in a six-minute test than those who were given the placebo.

“We believe we have already demonstrated in just a month of dosing a profound effect,’’ said Bill Hinshaw, chief executive of Axcella. His firm developed the experimental drug to combat the muscle weakness and fatigue associated with the fatty liver disease nonalcoholic steatohepatitis, or NASH. The drug is undergoing a separate study for NASH.

Axcella hopes to soon share data from the long COVID study with the US Food and Drug Administration and the UK Medicines and Healthcare Products Regulatory Agency to determine the next step.

Dr. Margaret Koziel, chief medical officer for Axcella and the former director of clinical research at UMass Medical School, said that if regulators approve the drug for long COVID, it’s unclear how long patients would take it. It’s possible the medicine would be used like antidepressants often are, for a few months until patients recover, but it could be prescribed for longer.

“This is a complicated disease,’’ she said of long COVID. “It’s not a single problem.’’

Dr. Jason Maley, head of Beth Israel Deaconess Medical Center’s long COVID clinic, examined the data at Axcella’s request and said the experimental drug is “the first pharmaceutical agent to demonstrate improved outcomes for patients with long COVID’’ in a clinical trial. The results, said Maley, whom the company paid for his review, suggest that the drug “may play an important role in the long-term treatment’’ of the disorder.

Two recent publications from the Journal of the American Medical Association found that 10 to 30 percent of people who had COVID-19 reported at least one persistent symptom up to six months after the virus left their bodies.

Axcella, which is publicly traded on the Nasdaq, was founded in 2011 and has about 70 employees. Like many decade-old biotechs, it has yet to get a drug approved.

Jonathan Saltzman can be reached at jonathan.saltzman@globe.com.