Biogen Inc. shares /zigman2/quotes/201531540/composite BIIB -2.89% slid 3% Friday, after a STAT report that the acting head of the U.S. Food and Drug Administration is calling for a federal investigation into her own agency's approval of a treatment for Alzheimer's disease, a month after the decision irked lawmakers, doctors and pubic health advocates and led members of an independent committee associated with the FDA to step down. In a letter, Acting Commissioner Janet Woodcock asked the Office of the Inspector General to investigate interactions between FDA staff and Biogen in the run-up to the approval of the drug Aduhelm, STAT reported. The agency cited reporting by STAT that said FDA officials had worked closely with Biogen to get the drug to market. "There continue to be concerns raised ... regarding contacts between representatives from Biogen and FDA during the review process, including some that may have occurred outside of the formal correspondence process," Woodcock wrote in a letter to acting Inspector General Christi Grimm. On Thursday, Biogen announced an updated label for Aduhelm that limited the patients who qualify for the drug to those with "mild cognitive impairment or mild dementia stage of disease" rather than a broader group of people diagnosed with dementia because that is the group of people who were studied in the clinical trials. The FDA has been criticized for approving Aduhelm with what some consider is questionable clinical data and Biogen for how the drug is priced, at $56,000 for a year of treatment. Biogen shares have gained 47% in the year to date, while the SPDR S&P Biotech ETF /zigman2/quotes/205950134/composite XBI -2.72% has fallen 5% and the S&P 500 /zigman2/quotes/210599714/realtime SPX -2.27% has gained 15%.