In 2003, he was diagnosed with basal cell carcinoma, a type of skin cancer. By then, Canada had legalized cannabis for medicinal purposes, but it was hard for Simpson to find a doctor who supported his use. A 1975 study in the The Journal of the National Cancer Institute showed how cannabis and its compounds inhibited tumor growth in mice, inspiring him to create RSO.
He has always maintained that applying RSO to his cancer growth and leaving it bandaged for four days cured him of cancer, but this has not been independently verified.
Simpson began producing the oil en masse and distributing it to thousands of patients for free. In 2009, his property was raided by the Royal Canadian Mounted Police and thousands of cannabis plants were confiscated.
To escape legal persecution, Simpson now lives in Croatia and maintains a website for his RSO recipe, dosing guide, and books. In 2018, he suffered a stroke and has since receded from the public but continues to advocate for RSO and medical cannabis.