TORONTO -- At 13, Christie was smoking pot daily. In time, she began dabbling in cocaine. But it wasn't until she was prescribed Percocet by her dentist following wisdom teeth removal in her early 20s that she began the long spiral downwards into a full-fledged opioid addiction that took over her life.

"I really liked that -- the way it made me feel," says Christie, who asked that her last name not be used for privacy reasons.

She subsequently received another Percocet prescription to treat abdominal cramps and "kept going back to my doctor for refills."

"And then I wasn't getting enough for how much I wanted to do," she says of the pills. "So what I did is I stole the prescription pad ... I took the script that she gave me and just copied it."

Christie got caught when a pharmacy checked with her doctor, who immediately dismissed her as a patient.

"When it started, it was medical, it was to help me with the pain management," she says.

But the medication had another alluring benefit.

"I never really felt like I had fit in, and the effects that the drugs gave me was that I did, like I could be that person that I wanted to be."

Christie was able to score Percocets from a street dealer in her southwestern Ontario hometown. But when she moved to Vancouver, "percs" weren't available, so she turned to OxyContin, a slow-release opioid that could be crushed for snorting or smoking to produce an instant high.

Living in Vancouver and working at a high-paying job in the Yukon two weeks per month, Christie continued to use whatever drugs she could get her hands on, though "Oxys were always the thing that I wanted the most."

When the manufacturer pulled OxyContin from the market in 2012 in favour of more tamper-proof OxyNeos, her West Coast dealer suggested she try a new pill that "looked just like the green Oxys."

"So I did -- and I really liked the way that made me feel," says Christie, her eyes growing wide at the memory.

The Oxy look-alikes turned out to be illicit fentanyl, a synthetic narcotic 100 times more potent than morphine that has led to an epidemic of overdose deaths, particularly in B.C.

By then in her early 30s and having lost her job, Christie was desperate to stop downing the eight to 10 fentanyl tablets that were costing her $200 to $300 a day.

"I had built up such a tolerance. I overdosed twice where I had to be Narcan-ed," she says in reference to the medication used for emergency treatment of opioid overdoses.

"I remember trying really hard to try to stop. I remember not wanting to have to go through that life anymore."

She went to a doctor and was put on methadone, an opioid-replacement drug used to help people manage their addiction or wean off their habit by keeping withdrawal symptoms at bay.

"But I couldn't stay sober, even with the methadone. I kept wanting to go back to doing the fentanyl."

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Just why do opioids -- painkillers that run the gamut from codeine and morphine to oxycodone and the fentanyl patch -- lead to dependence, or in other cases become so stubbornly addictive?

First, let's look at how they work.

When ingested, the drugs bind to opioid receptors on the surface of cells, primarily in the central and peripheral nervous system -- much like keys fitting into a lock -- and block the transmission of pain messages to the brain.

But opioids also work on multiple systems in the body, including the one that controls emotions, giving rise to feelings of pleasure, relaxation and often euphoria.

What's insidious about the medications is that they create physical dependence: over even a short period of time, patients build up a tolerance and need increasingly higher doses to achieve the desired effects, says Dr. David Juurlink, head of clinical pharmacology and toxicology at Sunnybrook Health Sciences Centre in Toronto.

"Let's say I put you on opioids for a week or two and you suddenly stop them," he says. "You would feel very unwell. The symptoms vary from person to person, but they include pain -- generalized pain, pain in the abdomen, pain in the legs.

"When you resume the drug, you feel better. And so you can see how a patient would construe that as evidence of effectiveness and even ongoing need for the drug, even after the pain-relieving effects have waned with time."

While short-term use of opioids can be a godsend for people with acute pain related to cancer, accidental injury or following surgery, many doctors now realize the narcotics aren't really beneficial for long-term treatment of chronic conditions such as arthritis, nerve damage or back pain.

"In fact, they cause pain," says Dr. Mark Ujjainwalla, co-founder and medical director of the opioid addiction clinic Recovery Ottawa. "So you get this hyperesthesia thing," he says. "It's actually a paradox. So the more opioids you get, the more painful everything is."

Physical dependence is pretty well a given for the hundreds of thousands of Canadians who have been prescribed an opioid, most of them for chronic pain.

But some people slide over the line into addiction, in which their craving for the physical and psychological high the drugs provide consumes their every waking hour -- despite the often life-shattering harms.

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Before she lost her job in Yukon, Christie's life appeared idyllic.

"I had the nice condo, I had the good boyfriend, I travelled around the world," she recalls. "My life externally was really wonderful. But inside I felt like everyone had something that maybe I didn't have.

"I couldn't cope, everything was overwhelming. My life was really unmanageable."

Opioids, primarily black-market fentanyl pills, were the solution.

But she and her boyfriend, also an addict, couldn't keep up financially with their insatiable need for the drugs.

"At the time, we had sold pretty much everything we had and we were manipulating people, lying to people, stealing, doing whatever we could to support our addictions, which were very expensive."

Eventually, the pair split up. Christie could no longer afford fentanyl, so she starting smoking, then shooting, heroin. She lost her condo and ended up homeless, finding shelter in a crack house in Vancouver's Downtown Eastside, crashing on the couch of a sex worker.

"I would wake up every day just thinking 'This isn't my life' ... I would promise myself 'I'm not going to use today,' and within five minutes I'd feel just the tiniest bit dope sick and I would call my dealer."

Dope sickness -- the result of withdrawal -- doesn't only cause pain, but also nausea, vomiting and diarrhea. Christie also had violent temperature swings -- one minute she was shivering with cold, the next soaked in sweat.

"It's like the worst flu you've ever had in your life -- times 20," she says. "And then you have restless legs. You can't sleep. I couldn't eat.... The restless legs were really, really bad for me. I couldn't stay still.

"So I would do anything not to feel that way."

After four months on the street -- except for occasional stints in detox clinics where she tried and failed to get clean -- Christie in desperation phoned her family, whom she hadn't been in touch with for years. They thought she was dead.

Her father flew to Vancouver and brought her home, where she went through "two weeks of hell" as she quit the drugs cold turkey. Ten days later, she entered a month-long addiction treatment program.

That was in the fall of 2015.

Now 35 and living in Toronto, Christie has been sober for about 18 months. She gives much of the credit to Cocaine Anonymous, a 12-step support program for those struggling to overcome their reliance on drugs or alcohol.

As part of her recovery, she has worked to make amends to her family and others whom she hurt because of her addiction.

Long drawn to yoga, Christie now teaches the discipline and also works "with other women like me" at hospitals and institutions, as well as leading or speaking at meetings of Cocaine Anonymous.

"So I started giving back, and that is when I really started to grow," she says, her face reflecting an inner contentment. "About halfway through my amends, I started feeling happiness like I'd never felt before."

For the first time, she is able to build relationships with her family and have "real friends in my life."

"I live a spiritual life now. I'm free."