BOSTON -- Heroin abuse in the city has stabilized in the past year, but alcohol remains a major substance of abuse, according to a new report released by the Boston Public Health Commission Friday.

The report, Substance Abuse in Boston, shows that the numbers of calls about heroin to substance abuse help lines and the number of people seeking treatment for heroin abuse have fallen. The data collected also shows that alcohol as a primary drug in treatment admissions accounted for 36 percent of total treatment admissions and 34 percent of all abuse help line calls, second only to heroin in 2006.

"We're not sure exactly why we're seeing some indicators on heroin abuse improve, but we hope that we are seeing a trend that will continue," said John Auerbach, executive director of the Boston Public Health Commission.

That trend, however, according to data in the report, indicates that the average age of people seeking treatment for heroin addiction is dropping, which Auerbach said "points to a higher need for enhanced prevention and treatment efforts targeted at young people."

"I'm encouraged by the report, but we can't be complacent," Mayor Thomas M. Menino said. "Heroin is still a major problem in our city and in our region, but I believe that our approach in Boston is having an impact."

According to Menino, the election of Gov. Deval Patrick, the appointment of Dr. JudyAnn Bigby as state Health and Human services secretary, and Auerback as the next public health commissioner for Massachusetts will help contribute more active leadership in fight against substance abuse.

"We need to redouble our efforts to focus on prevention and on increasing treatment options for those who are caught in the cycle of addiction," Menino said.

Data from the report showed that several different classes of drugs such as oxycodone and cocaine abuse have stabilized but remain at high levels throughout the state.

Marijuana abuse has remained at a fairly stable level for the past five years; however, the number of marijuana drug arrests increased in 2005.