U.S. regulators have clarified the role of a decades-old drug once presented as a possible breakthrough for autism, significantly narrowing its approved use after reviewing the available scientific evidence.

The Food and Drug Administration announced that leucovorin, a medication historically used to reduce the side effects of chemotherapy and to treat certain forms of anemia, will be approved specifically for cerebral folate deficiency, a rare neurological disorder that affects how the brain processes folate, a form of vitamin B. The condition can produce developmental delays and symptoms that overlap with autism.

The decision marks a shift from earlier expectations that the drug might benefit a much larger population of patients with autism spectrum disorder.

Health officials had previously highlighted the medicine as a promising avenue for addressing symptoms in some children, particularly those suspected of having abnormalities in folate metabolism.

Federal health authorities say the revised approval reflects a careful review of existing research.

While some early studies suggested the drug might improve communication or behavioral outcomes in certain children, the evidence remains limited and has not yet met the standard required for approval as a general treatment for autism.

Cerebral folate deficiency is considered extremely rare, with fewer than one in a million people estimated to be affected.

In this disorder, the brain does not receive adequate levels of folate despite normal amounts circulating in the bloodstream.

Treatment with leucovorin can help restore folate levels in the brain and address neurological symptoms in affected patients.

The clarification also comes after a surge in interest in the drug among families seeking new therapies for autism.

Prescriptions for leucovorin increased significantly following earlier announcements highlighting its potential benefits, reflecting strong demand among parents searching for treatments that might improve speech and social development.

Medical specialists emphasize that autism remains a complex neurodevelopmental condition with diverse underlying causes.

Many researchers believe that treatments targeting specific biological pathways—such as folate transport in the brain—may help small subgroups of patients, but broader solutions will likely require extensive research and multiple therapeutic approaches.

Regulators say further clinical studies will be necessary to determine whether leucovorin could play a larger role in autism care.

For now, the drug’s approval remains limited to patients diagnosed with cerebral folate deficiency, while doctors retain the option of prescribing it off label in individual cases based on clinical judgment.