The U.S. Food and Drug Administration on Thursday alerted patients and healthcare professionals about the withdrawal of Pfizer's sickle cell disease treatment Oxbryta.

Pfizer’s decision Wednesday to pull its sickle cell pill off the market because of safety concerns shocked advocates and doctors, leaving many searching for answers and scrambling for ways to notify their patients about a drug that has long divided the community.

Pfizer is withdrawing all of its sickle cell disease drug, Oxbryta, worldwide because of safety concerns. Pfizer ( PFE) shares declined Thursday after the drug maker recalled all lots of its sickle cell disease medicine worldwide because of concerns it could lead to severe pain and possibly death in patients.