Hank McCoy's Serum is a serum developed by Hank McCoy in order to suppress the mutant gene. Originally derived from the cells of Mystique, the original variation accelerated Hank's mutation, giving him a more animal-like appearance. Later versions were more successful, allowing Hank to remain in his human form and even taking away Charles Xavier's powers, but allowing him to walk again.
X-Men: First Class
Hank is able for to create a prototype serum using Mystique's blood as its foundation, but instead of suppressing his mutation's cosmetic cells, it enhanced them, resulting in him looking like a blue, humanoid beast.
X-Men: Days of Future Past
While never seen using the serum, Hank explained after his serum wore off and everyone saw his mutation that he left his meds at home.