New London — Since most addiction starts in the teenage years, the most effective way to combat the rising rates of heroin and prescription opioid addiction it to “stop teenagers from using anything,” one of the region’s leading substance abuse prevention experts told the New London Rotary Club Thursday.

“We need to keep the vulnerability envelope of teens closed for as long as possible,” said Karen Fischer, a longtime mental health counselor currently working for the Child & Family Agency of Southeastern Connecticut at the Lymes’ Youth Service Bureau on a five-year substance abuse prevention grant.

Fischer’s presentation during the civic group’s weekly meeting came as southeastern Connecticut is confronting a recent spike in heroin and prescription opioid overdoses and deaths.

She emphasized the physiological conditions in teenage brains that predispose them to addiction if they begin abusing substances including alcohol, marijuana and prescription opioids, and that any introduction to them before the brain is fully developed at age 25 can do lifelong harm.

“If a teen is repeatedly using substances, they are repeatedly laying down pathways for really liking the substances, and those pathways are very, very hard to undo,” she said. “Addiction is a disease of the brain. If teenagers start using substances and are not stopped early on, they are in trouble.”

Before she began her talk, one of the members of the club read a letter recently posted on Facebook by a local father whose 22-year-old son died of an overdose on Jan. 31.

The letter, which began, “Dear heroin, I hate you…” was written by the young man while he was in a recovery program and recounted how addiction had ruined his life.

Two other members read first-person accounts of a young woman who abused alcohol as a teenager and later became addicted to heroin, and of a young man who recognized in time that he was becoming addicted after using prescription opioids for a broken leg, insisting his girlfriend get rid of the medications before he took more.

“Two to three weeks of continued use can lead to addiction,” Fischer said.

Some people, she said, are more genetically predisposed to addiction than others, but since there is no way to know that until it’s too late, the most prudent approach is to keep teens from abusing any substance.

In Lyme and Old Lyme, she said, a campaign targeting teens and parents has helped to reduce substance abuse.

From 2009 to 2013, she said, surveys of youth have shown a 46 percent reduction in marijuana use and a 50 percent drop in alcohol use.

In the most recent survey in January, she said, 13 percent of high school students said they had used alcohol in the last 30 days, and 13 percent said they had used marijuana in that same time period.

She advocated for “a change in cultural norms” around alcohol use, so that parents refuse to let their own teenage children or their friends drink in their homes.

“We need to move toward no use in our families, and make that stick,” she said.

She also alerted members of the club to a bill pending in the General Assembly that would legalize the recreational use of marijuana.

“Call your legislators today and tell them how you feel about this,” she said, responding to one member’s criticism of the proposal.

j.benson@theday.com

Twitter: @BensonJudy