Jeff Conaway, the former star of Taxi and Grease who has a history of substance abuse problems, is currently in critical condition at an Encino, Calif., hospital after a suspected overdose, reports the Los Angeles Times. The 60-year-old actor has pneumonia and remains unconscious. In 2008, Conaway appeared on two seasons of Celebrity Rehab with Dr. Drew to get help for his addiction to cocaine, painkillers, and alcohol.