From his early days of ministry in Pennsylvania to his postings in the West Indies and the nation’s capital and finally in Boston, Cardinal Sean O’Malley has had a front-row seat on the ravaging toll marijuana and other drugs have taken on people near and dear to him, particularly the young.

For O’Malley, fight against Question 4 — the ballot measure that would make pot legal in Massachusetts — is personal.

“In my long years of ministry,” the cardinal told me yesterday, “I have seen so many lives destroyed by marijuana and other drugs.”

He recalled the promising young man from the inner city of Washington, D.C., whose life spiraled downward after he began smoking marijuana. He eventually started using other drugs, dropped out of school, became homeless and died young.

The young man came from a loving, church-going family, had a supportive parish and had even landed a scholarship to a Catholic high school.

But, O’Malley said, “his life came apart and it all began with marijuana.”

“He was a very bright and talented kid. It destroyed his life. And that’s just one of many that I’ve seen this happen to,” O’Malley said.

“He was a lot of fun. He was doing well in school. He had leadership qualities. I know he would have gone far if it hadn’t been for the involvement with marijuana and drugs that brought him down,” he said.

Tragic stories like that one are driving O’Malley in his very public battle against Question 4. He has appeared in a series of web videos speaking out against it. He’s also put the archdiocese’s money where his mouth is, donating $850,000 to a group campaigning against legal pot.

The biggest eye-opener about marijuana’s potentially devastating impact came during the 10 years O’Malley spent in the West Indies, where he saw weed ruin lives and families, particularly the Rastafarians.

He saw young people, addicted to “ganja,” who couldn’t hold down a job or create a family, because they’d built their lives around the drug. Many died young. The damage to their health and minds was obvious, he said, “and that was exclusively marijuana.”

That experience made a huge impression on him and convinced him that he had a duty to warn others about the dangers of drugs, in particular marijuana, which is often billed as a harmless, recreational pastime.

O’Malley’s first ministry was at a Pennsylvania prison, where he worked with inmates and their families. “The prisoners were all very young. So many of them became involved with marijuana, dropped out of school and couldn’t get a job,” he said. “In many cases, it was a gateway drug for other kinds of drugs. In other cases, it was combined with alcohol to really destroy their lives.”

O’Malley spent 20 years working at inner-city parishes in Washington, D.C. “Most of the funerals that I had were murders,” he recalled. “Many of those were caused by being involved with drugs. I buried a number of people who had died as a result of their addictions.”

Here in Boston, O’Malley says he “constantly” hears from parents whose son or daughter “has become addicted, we didn’t realize it, they dropped out of school, they can’t hold a job. Their whole life is on hold because of this.”

Not long ago, the nephew of a priest committed suicide after becoming addicted to drugs. And as so often happens with addiction, the cardinal said, “His psychological problems began with marijuana.”

With polls showing that Question 4 will likely pass into law, O’Malley says he’s particularly worried about children. Marijuana is much more potent today, for starters, and packaged as a cookie, candy or brownie, it’s only going to be more enticing to kids.

“There should be a great effort on the part of society to let people know about the dangers of this drug,” O’Malley said. “There’s so many side effects and particularly in the lives of young people. The sooner you start smoking marijuana, the more likely you will become addicted to it.”