Alcohol, marijuana, illicit drugs, nicotine, and compulsive behaviors involving pornography or sex can gradually interfere with health, relationships, work, judgment, finances, or the law — and the line between use and a problem isn't always obvious. These struggles are common: the 2024 National Survey on Drug Use and Health found that 48.4 million Americans age 12 and older — 16.8% of the population — met criteria for a substance use disorder. Substance use often coexists with depression, anxiety, trauma, ADHD, bipolar illness, or sleep problems, so understanding what the substance is doing for a person — relieving anxiety, aiding sleep, easing loneliness — is often key to changing it. Recovery can mean different things for different people, from harm reduction to sobriety, and relapse is treated as information rather than failure.