In 1979, Clark left government-funded research and began private consulting and her own research. From 2002 until her death she operated the Century Nutrition health clinic in Tijuana, Mexico, where her focus was primarily on late-stage cancer patients. Clark and her son Geoff separately owned businesses her patients and others used, including a restaurant, her self-publishing company, and a "self-health" store that sold her inventions. Her son continues to operate the self-health store. She published several books, including "The Cure of All Cancers", "The Cure for HIV/AIDS" and "The Cure For All Diseases". According to civil court records, her books generated over $7 million in sales by 2002, although Clark disputed this figure.