Sarepta Therapeutics Inc.’s volatile stock tumbled 27 percent on Thursday amid fresh doubts regulators will approve its treatment for a deadly muscle disease.

Analysts said a simplified process to give patients access to experimental drugs, outlined Thursday by the Food and Drug Administration, may make it less likely the federal agency will OK Sarepta’s drug to treat Duchenne muscular dystrophy. An FDA advisory committee has recommended rejecting the drug, called eteplirsen, which is aimed at treating a muscle-wasting genetic disorder that strikes one in every 3,500 boys.

Shares of the Cambridge company fell $5.70 to $15.71.

The statement from FDA Commissioner Robert Califf on streamlining the process doctors use to request investigational drugs for so-called compassionate use by their patients didn’t apply specifically to Duchenne. But the expanded access guidelines represent “a potential way out’’ for the FDA, according to a research report issued by biopharma analyst Simos Simeonidis at investment bank RBC Capital Markets.

The agency is facing intense pressure from patients and their advocates to approve a drug questioned by some medical experts, but the new guidelines could give more patients access to the medicine — which has few safety risks — even if it isn’t on the market.

Sarepta’s shares climbed 27 percent on May 25 when the FDA postponed its long-awaited decision on eteplirsen, with many investors interpreting the delay as a sign the agency might approve it. Eteplirsen is designed to treat about 13 percent of Duchenne patients who have a specific genetic mutation.

FDA officials declined to discuss their timetable for ruling on the drug, saying they were prohibited by law from discussing pending drug applications.

Robert Weisman can be reached at robert.weisman@globe.com. Follow him on Twitter @GlobeRobW.