Editor's note: This is part of an ongoing series on opioid overdoses in the Northland.

To read the main story, go here.

When someone is addicted to heroin, the sole purpose of their life becomes acquiring and using the drug.

"It's like nothing else matters - your kids, everything you've gained, it doesn't matter," said Patricia Halder, a recent drug court graduate who is celebrating more than 19 months sober. "It's like, 'I'm going to get this, and do it, and that's it.' It's like tunnel vision."

Unique to the opioid epidemic is that the most desirable effect someone can reach on drugs such as heroin is a margin of error away from the lethal effect, said Dr. Charles Reznikoff, the addictions medicine expert at Hennepin County Medical Center.

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"The most euphoric feeling, the most sort of calm, placid, pain-free a person can be on opioids is at a very high dose," Reznikoff said. "If you accidentally exceed what you need for that calm, placid euphoria, the next thing that happens is an overdose.

"That is not true of other drugs. It's not true of tobacco, marijuana, even alcohol."

The likelihood of a person becoming addicted varies, depending on a combination of genetic predisposition and environmental factors - a living situation that favors drug use, having friends or family who engage in the behavior, or adverse childhood experiences.

People with opioid addiction have a 25 percent risk of death - the same as someone with metastatic colon cancer - Reznikoff said. After someone lives through an overdose, often with the help of Narcan, their chance of dying doubles.

Many overdoses occur after a period of abstention, sometimes due to incarceration or treatment, Reznikoff said. After losing tolerance, the user often returns to using the same amount they did before. It's a scenario which can be ripe for an overdose.

And increasingly, powerful synthetic opioids like fentanyl are showing up in the heroin supply - making even a small dose a killer.

"There's a high amount of uncertainty about what people are using," Reznikoff said. "That leads to the fact that every time you're using opioids you say, 'I don't know how strong this is. I want to use enough to feel good, but if I use even a little bit too much I could die.' Every use has to have that calculation with it."

You can read an introduction to the series here.

For treatment resources, go here.

To view an interactive map of public overdoses, go here.