Answer by taking a quote from the following article:

Celebrity Rehab with Dr. Drew, later called simply Rehab with Dr. Drew, is a reality television show that aired on the cable network VH1 in which many of the episodes chronicle a group of well-known people as they are treated for alcohol and drug addiction by Dr. Drew Pinsky and his staff at the Pasadena Recovery Center in Pasadena, California. The first five seasons of the series, on which Pinsky also serves as executive producer, cast celebrities struggling with addiction, with the first season premiering on January 10, 2008, and the fifth airing in 2011. The sixth season, which filmed in early 2012, featured non-celebrities as treatment subjects, and the series name shortened to Rehab with Dr. Drew. Season 6 premiered on September 16, 2012.

According to a December 2009 New York Times article, Drew Pinsky, who was alarmed by tabloid portrayals of addiction as an indulgence of the rich and famous, and a group of independent producers, approached VH1 with a proposal for a reality television series which would authentically depict addiction, as a sort of media intervention. According to executive producer John Irwin, casting for the first season was the most difficult, as the representatives of the celebrities who had been arrested or had publicized bouts with addiction refused to speak with him and the other producers. The process became easier after the first season aired. Actor Tom Sizemore, for example, who was cast for Season 3, had been sought after since Season 1. Producers have reportedly offered actress Lindsay Lohan six figures to appear on the show. Pinsky, who focuses on the treatment side of the production, is not usually involved with casting, though he reportedly visited Rachel Uchitel personally in order to convince her to join the fourth-season cast. A multitude of cameras are employed, which film twenty-four hours a day, seven days a week, during the 21-day treatment cycle. Because some dramatic incidents occur early in the morning when the camera crews are not present, automated cameras are mounted all over the clinic to capture them. The exception to this are the bathrooms, which nonetheless are equipped with microphones to monitor unusual sounds, such as patients attempting to use drugs. In addition to receiving the free treatment (which would normally be worth approximately US$50,000-60,000), the patients receive a salary for their appearance on the series, which is prorated, and distributed once a week as an incentive to stay. Celebrity Rehab spun off two other shows. The first spinoff is Sober House, which depicts Rehab alumni living for 30 days at a sober living facility, as an interim step for recovering addicts between the completion of rehab and their eventual return to their old life. The second spinoff is Sex Rehab with Dr. Drew, in which Pinsky and his staff treat celebrities for sexual addiction. In May 2013, Pinsky announced that season six was the final season, citing the criticism leveled at him following the relapse and death of cast members, saying, "I'm tired of taking all the heat. It's very stressful and very intense for me. To have people questioning my motives and taking aim at me because people get sick and die because they have a life-threatening disease, and I take the blame? Rodney King has a heart attack and I take blame for that? It's just ridiculous." Pinsky insisted that his practices depicted on the show were the same ones he, Bob Forrest and Shelly Sprague engaged in for decades, with the sole difference being the presence of cameras. Despite his decision not to continue the series, Pinsky stated a number of people achieved sobriety due to Celebrity Rehab who would not have attained it without the show, and cites as the biggest success story Sizemore, who struggled with Pinsky's program for 10 years before finally achieving sobriety after being on the show.

how many seasons was dr. drew on?
season six was the final season,