Pop icons Carrie Fisher and George Michael died this week after struggling with drug abuse for decades. While it was not immediately clear whether addiction played a role in their deaths pending autopsy results, drugs such as oxycodone, hydromorphone and heroin can cause cardiac arrest. What's more, drug abuse of any kind can result in an earlier death. 

Drug abuse involves the continued use of alcohol, illegal drugs or over-the-counter drugs with negative consequences. Drug addicts don't lack willpower or morals. They suffer from a complex disease that can affect their brain and make it harder to quit. Even if someone quits a drug for years, they can easily relapse because of drug use disorders. 

Repeatedly using drugs can further fuel urges to do more drugs, making it harder to resist, by flooding the brain with the chemical messenger dopamine. It can make someone feel like they need drugs to thrive. That "high" drives the repeated drug use, according to the National Institute on Drug Abuse.

Long-term drug use can affect learning, judgment, stress, memory and behavior. As with diabetes or asthma, there isn't a cure. Treatment must be ongoing based on a patient's changing needs and response. 

Sometimes a person's genes can make them more vulnerable to addiction. Gender, ethnicity and other mental disorders also increase risk for drug use and addiction. Peer pressure, physical and sexual abuse, early exposure to drugs and parental guidance also can result in drug use.

Commonly abused substances include alcohol, cocaine, marijuana and prescription drugs. 

Addiction can result in other medical issues, including stroke, cancer and mental disorders. Drug abuse can also damage or destroy nerve cells. Tobacco use, for example, contributes to an estimated 5 million deaths worldwide each year. Opioids such as heroin killed a record 33,000 people last year.

To get help, addicts should be prepared to stop using drugs and once again become a productive members of their family or society. Counseling and medications can help people quit their bad habit. 

Fisher, for one, at times made light of her former drug battles. Fischer, 60, died Tuesday after a heart attack.

"I never used my fingernail for drugs. I used dollars or tiny spoons like any other respectable former drug addict," she once wrote on Twitter.