CHICAGO — For some, opioids are an effective, non-addictive pain reliever. But about 10 percent of the population is hard wired for addiction, experts say, and even casual use can quickly turn into something even more serious.

As the drugs make their way through the body, they bind to natural opioid receptors on nerve cells. In the brain, they take over an area responsible for pleasurable, rewarding and reinforcing experiences.

Food and sex spark the same feelings – but for very brief periods. Opioids, on the other hand, stimulate the nerves with unrelenting force – not for seconds, but for minutes, hours or days.

“What happens is heroin or other opiate drugs highjacks this entire system,” Northwestern Medicine drug researcher Dr. Richard Miller said.

In the midbrain, the pleasure center of the brain, nerve cells aren’t accustomed to such an assault. So they actually rewire themselves and as a result, the brain sends out signals telling people they absolutely have to use. That’s why people will try to seek them out, even turning to illegal drugs if necessary. Cocaine has a similar effect – but it’s not as widely available as opioids.

Recovering addict Ryan Johnson says he feels powerless to ignore the deafening voice of addiction.

“It’s literally like if I don’t do this, the world’s gonna end. I am gonna lose my life. Even though I am losing my life [by using]. Doing drugs it’s the craziest thing,” Johnson said.

But Dr. Miller said it’s not crazy.

“The brain of an addict is no longer like the brain of a normal person, and the addict really is compelled to do this and cannot help doing it any longer,” Dr. Miller said.

But there is hope: after years of attempting and failing recovery, Ryan has been clean for a year, and even met his wife Megan in a program.

Their daughter – and a letter written on her behalf by a family member – serve as a constant reminder of the huge cost of failure.