Local retirement officials voted Wednesday to take away the pension of Joseph Deignan, a former Watertown police sergeant who faces 90 days in prison on federal drug charges. Deignan, a Framingham resident, pleaded guilty in August to stealing a driver’s license while on the force in 2010, and using it to obtain forged prescriptions for oxycodone and other controlled drugs, according to the office of US Attorney Carmen Ortiz. Deignan was charged with unlawful possession of a controlled substance by fraud, and fraud in connection with identification documents, the prosecutor’s office said. State retirement law states that a public employee’s pension can be taken away following a conviction on charges related to his or her official position. Deignan, who served on the police force for 32 years, has been collecting an annual pension of $55,731 since his retirement in February 2012, officials said. The Watertown Retirement Board voted Wednesday to cease the pension retroactively to when he was sentenced in mid-November. However, officials said in a statement that the law seemed too stringent, noting they wished to take other factors of Deignan’s career and life into account. They said they plan to share their opinion on the need to amend the law with the Public Employee Retirement Administration Commission and the Legislature.