TORONTO — An aging construction worker arrived quietly in the building’s basement, took his seat alongside three other men and struck his lighter below a cooker of synthetic heroin.

A woman, trained to intervene in case of an overdose, placed a mask over her face as his drug cooked and diluted beneath a jumping flame. He injected himself, grew still and then told of the loss of his wife who died alone in her room upstairs — an overdose that came just a few months before this social service nonprofit opened its doors for supervised injections.

“I don’t want to say, ‘What if, what if,’” said the 52-year-old man, known as Gordie. He said his wife, Carol, had talked about kicking her habit before her death in September. “It never happened,” he said.