A GMC panel says his actions were irresponsible, and found him guilty of serious professional misconduct.
Irwin, a former UN medical director and head of the Voluntary Euthanasia Society is fighting the case to highlight his call for a change in the law.
He openly admitted traveling to the Isle of Man in October 2003 with around 60 temazepam sleeping pills to assist fellow campaigner Patrick Kneen to end his life.
Kneen, who had prostate cancer, was too ill to take the medication and slipped into a coma, dying a few days later without Irwin's assistance.
Police took the decision not to proceed with the case, but he received a caution for possessing the Class C drug with intention to supply.
Irwin obtained half of the pills under his own name at the Moss Pharmacy, in Cranleigh near his home.
Professor Ken Hobbs, head of the GMC's fitness to practice panel, says Dr Irwin's actions in writing the prescription in his own name was "an act of deception".
