What are illegal drugs? Are they all addictive? Drugs are substances that affect the way your mind and body functions. Drugs can affect the way your brain works, influence your emotions and behaviours, and even your understanding and senses. If a drug is classified as ‘illegal’ this means it’s forbidden by law, due to the harm it can cause to your health, your life, or the life of others. There are many factors which influence whether someone becomes dependent (addicted), but regular drug use is a common factor. To learn more about different types of drugs, and drug related terms, check out our glossary and drug information factsheets.