In 1988 Thorburn was fined £10,000, had two ranking points deducted, and was banned for two ranking tournaments, by the World Professional Billiards and Snooker Association. The Association's disciplinary committee had decided that Thorburn had brought the sport into disrepute, as a drug test that he took at the 1988 British Open showed that he had "minute traces of cocaine in his urine sample." He compiled another maximum break in the 1989 Matchroom League, during a match against White.