Carrie Fisher had multiple drugs in her system when she died after suffering a heart attack in December, People reported today, including heroin, cocaine, and alcohol. A report from the Los Angeles County coroner’s office stated that doctors were not able to “establish the significance” of these substances with regard to the cause of the actress’s death, which was earlier attributed to sleep apnea and other undetermined factors.

Regardless of whether these drugs played a direct role in Fisher’s passing at age 60, the revelation does highlight the actress’s long-time struggle with drug addiction—something she spoke openly about for many years. In an exclusive statement to People, Fisher’s daughter, Billie Lourd, said that Fisher battled addiction and mental illness her entire life, and that “she ultimately died of it.”

“She talked about the shame that torments people and their families confronted by these diseases,” Lourd said. “I know my Mom, she’d want her death to encourage people to be open about their struggles.”

But why exactly are substances like illicit drugs, alcohol, and opioids so addicting—and why do they continue to ruin lives of people, of all ages, who try for so many years to get and stay sober?

For a better understanding, Health spoke with Kenneth Leonard, PhD, director of the Research Institute on Addictions at the University at Buffalo. Leonard did not treat Fisher, but he has helped many others with their own drug problems. Here’s what he wants people to know.

RELATED: 8 Times Carrie Fisher Shattered the Stigma on Mental Illness

Addiction causes long-lasting brain changes

Drugs and alcohol have the capacity to change the structure of the brain so that they become more appealing and more important to the user, says Leonard. “And these changes are long-lasting,” he says. “We don’t know if they’re ever reversed, or if they are, what kind of a time frame it’s in. The years of taking any of those drugs have implications for the way the brain develops long-term.”

There are psychological and social reasons, too

“People often have memories of when those drugs were positive for them—when they helped them through tough times or when they used them in positive situations,” says Leonard. “They don’t always remember all the negative situations.”

Many times, addiction is driven by a social component as well: “If you know people who have access to these drugs and you’re around them when they’re using them, that is certainly a temptation and an opportunity for willpower to fail.”

RELATED: How to Take Opioids and Not Get Addicted

Years of recovery doesn’t mean a person is in the clear

The longer a person is sober, the better his or her chances are of staying that way. But that doesn’t mean there aren’t still risks, even years after a person has last used drugs or alcohol. “People continue to experience those desires to want to take their drug of choice,” says Leonard.

“Sometimes they have lapses and are able to get back to a sober recovery, and sometimes they go on to continue using,” he adds. “What’s important is they get back to seeking help and get back to the sober recovery path they were on.”

Mental illness can play a role

People who struggle with psychological disorders—even if they’re being treated and are under control—can face additional challenges. “Sometimes symptoms can emerge and people feel like they need a little something,” says Leonard.

“In the case of depression or anxiety, it might make them feel better,” he says. “Or in the case of bipolar disorder, it might make them feel as though they can take this and not suffer the harms.” (Fisher spoke openly about her bipolar disorder.)

Beating addiction may be even more important as you age

Drug and alcohol abuse can have serious consequences whether a person is 18 or 80. But age-related changes to the brain and body can make it even more dangerous as a person gets older, even with low levels of drug use. The body may not be able to break down these substances as easily as it once did, and their effects could be compounded by other health issues, like heart disease.

In addition, Leonard says, people who relapse after being sober for years will have a lower tolerance to a drug than they used to. “If you are off these drugs for some time, the amount you need in order to feel something goes down—and so does the amount that will cause physical problems,” he says.

People with addiction aren’t weak

Leonard says it’s important to help people understand that, when someone relapses, it’s not because they’re weak or because they’re making a deliberate choice to harm themselves.

“We all experience temptations, like having one more cookie or just a couple of potato chips,” he says. “Just imagine if you were dealing with something that has such a powerful and profound impact on physical and psychological symptoms.”

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Beating addiction is hard—but it’s possible

This may paint a bleak picture, but Leonard says that people can and do stay sober after years of drug abuse. He encourages people who are struggling to seek help, which may involve medication as well as behavioral and psychological counseling.

For people who have beat drug addiction, he recommends “relying on relationships and the supportive mentors that you’ve built up, especially when you've been feeling like you could be tempted.” Seek out friends, family, and support groups that have helped in the past, he adds, and don’t wait until a relapse to talk about what you’re feeling. “Have them remind you of the positive things that have happened since you’ve been sober, as well as the fact that you’d be taking a tremendous risk by returning to substance abuse.”

In her statement recognizing her mother’s struggle, Lourd also encourages people to seek help, and to fight for government funding for mental health programs. “Shame and those social stigmas are the enemies of progress to solutions and ultimately a cure,” she wrote.