85 Fentanyl is a synthetic opioid that is much stronger than heroin and that has recently contributed to a significant number of overdose deaths. Ketamine was originally developed as an anaesthetic for humans and animals. It causes a feeling of detachment from reality (dissociation) and a sense of being outside one s body. ADDICTIONS Addictions are chronic conditions characterised by compulsive behaviours or consumption of substances which involve a physical and/or psychological dependency; the individual feels unable to stop the behaviour or use of the substance regardless of their harmful consequences. Drug addiction Drugs act on the central nervous system, causing psychological changes and health problems. Drug use leads to two types of dependence: physical, when stopping consumption causes physical disorders (anxiety, muscle pain, vomiting, tachycardia) known as withdrawal symptoms ; psychological, when the lack of drugs creates an overwhelming need to get them. An overdose occurs when a person consumes more of a drug than the body can process, leading to toxic effects and even to death. The most common types of drugs can be divided into the following broad categories. craving: voglia overwhelming: travolgente withdrawal: astinenza Stimulants increase the activity of the central nervous system, leading to unnatural energy and sometimes euphoria. The most common are cocaine, methamphetamine (Meth), and ecstasy (MDMA). Opioids are a class of drugs that are commonly used for pain relief but can lead to severe physical and psychological dependence. The most common are heroine (illegal drug derived from morphine), painkillers (which can be prescribed for pain relief but that can lead to opioid dependence), and fentanyl . 162 SCIENCE AND PHYSICS Hallucinogens alter perception, mood, and thought, often causing users to experience sensory distortions and violent behaviour. The most common are LSD, ketamine , and PCP (phencyclidine). Psychoactive drugs possess properties of multiple drug categories: depressant because they cause relaxation, stimulant because they increase alert, and hallucinogens because high doses can cause mild hallucinations. Cannabis and hashish belong to this category. Tobacco addiction Tobacco addiction refers to dependence on tobacco products, particularly cigarettes, due to the addictive properties of nicotine, a chemical found in tobacco. When smoked, nicotine rapidly enters the bloodstream releasing dopamine, a neurotransmitter associated with pleasure and satisfaction. Over time, the person may need to use tobacco regularly to avoid withdrawal symptoms, such as irritability, anxiety, and cravings. Tobacco addiction can cause serious health problems such as cancer, cardiovascular diseases, and respiratory issues. Alcohol addiction Alcohol addiction refers to a condition where an individual has a strong, compulsive desire to drink alcohol despite the negative consequences it may cause in their life. Just like nicotine, alcohol affects the brain s reward system, with the consequent release of dopamine which creates a feeling of pleasure or euphoria. Health risks related to alcohol addiction include liver damage, mental health issues, and cardiovascular problems.