WATERBURY, Vt. — Efforts to make OxyContin harder to abuse have succeeded in Vermont, the top state police drug investigator says. But that has come with one notable consequence: Addicts have been switching to an older scourge: heroin.

Statistics provide a glimpse of Vermont’s growing heroin problems. Deaths attributed in whole or in part to heroin overdoses averaged fewer than three a year from 2004 to 2010. There were nine such deaths last year, and six this year, according to figures from the state Health Department.