KAMPALA, Uganda — Pain is only the latest woe in John Bizimungu’s life.

Rwandan by birth, he has lived here as a refugee since his family was slaughtered in the 1994 genocide. A cobbler, Mr. Bizimungu used to walk the streets asking people if he could fix their shoes.

Now, at 75 and on crutches, he sits at home hoping customers will drop by. But at least the searing pain from the cancer that has twisted his right foot is under control.

“Oh! Grateful? I am so, so, so, so grateful for the morphine!” he said, waving his hands and rocking back in his chair. “Without it, I would be dead.”

Mr. Bizimungu’s morphine is an opioid, closely related to the painkillers now killing 60,000 Americans a year — a situation President Trump recently declared a “health emergency.”