When people are asked which substances they think are addictive, they usually mention illicit drugs. Legal substances like nicotine and caffeine don’t normally come to mind. But a 2007 study by Nutt et al  found that alcohol and nicotine, while both legal, are more addictive than MDMA* (ecstasy).
How did they determine the addictive potential of these substances?
They applied multi-criteria decision analysis (MCDA) modelling as a measuring tool. They scored 20 drugs on 16 criteria: nine related to the damage a drug produces in the individual and seven to injury to others. They reported that heroin, crack cocaine, and methamphetamine were the most harmful to individuals, while the first two and alcohol were the most damaging to others.
The Business Insider reports another ranking system, which divides the criteria into:
• physical harm—the likelihood of causing injury
• psychological/physical dependence—the intensity of reward, plus reliance on the substance
• social harm—effect on relationships; cost to the community and health care
Other determinants for substance abuse include its street value, how it affects the brain’s dopamine (‘happy’ hormone) levels, how much pleasure it gives , how severe its withdrawal symptoms are, and the speed or ease by which it induces dependence.
The world’s most addictive drugs according to the above criteria:
1) Heroin is addictive because it creates euphoria, blocks pain, and relaxes the body by releasing excess dopamine. When addicts quit, their bodies won’t produce dopamine for a while. So symptoms like feelings of pain, nausea, depression, and hallucinations occur, compelling users to return to their habit. This opioid synthesized from morphine tops the list because it’s accessible, affordable, and the gap between the dose one needs to get high and the dosage that causes overdose is minimal . It’s also lethal to a relapsing addict.
2) Cocaine, including crack and powder cocaine, prevents dopamine’s pleasure signal from turning off, leading to abnormally high levels. This promotes euphoria, shedding of inhibitions, alertness, and appetite suppression. Cocaine’s effects are intense but short-term, prompting users to snort, smoke, or inject more of it.
3) Nicotine, the main ingredient in tobacco products (including many e-cigarette formulations), is quickly absorbed by the lungs and the brain. The World Health Organization (WHO) estimates that there will be more than 8 million tobacco-related deaths by 2030. In the United States, addiction to nicotine is the most common.
4) Barbiturates, aka downers, were originally used to treat sleep disorders and anxiety by shutting down parts of the brain. Later, these depressants were used as anesthetics, as well as remedies for seizures and severe headaches. Low levels produce euphoria, but high doses can inhibit breathing. Addiction to barbiturates lessened when they got harder to obtain and newer drugs (fentanyl) became more accessible.
Other studies listing the top five put methamphetamines (crystal meth) and amphetamines (stimulant ADHD medications) in place of barbiturates. Later, benzos (benzodiazepines)—psychoactive drugs for anxiety and insomnia, like Valium and Xanax— overtook barbiturates in popularity.
5) Alcohol is similar to street drugs in that it also has mind-altering capabilities. It affects the brain’s reward pathways by greatly increasing dopamine levels. It’s life-threatening because of its potential for overdose. Its universal accessibility makes it harder for dependents to avoid it. The WHO’s 2012 report revealed that more than 3 million deaths that year were alcohol-related.
Learning about the above drugs and their effects on the brain and body can serve as a warning, help curb addiction, or prepare one to handle the condition before it becomes lethal. One should be particularly careful of prescription drugs (especially painkillers), as their addictive potential is not as obvious as that of street drugs. Those who have fallen prey to the lure of the former without realizing it—because these drugs are not normally associated with addiction stereotypes—find out too late that they’re in too deep and need professional help. This is why early education can be life-saving.
* MDMA: abbreviation of the chemical name methylenedioxymethamphetamine.
 Nutt, DJ, King, LA, and Phillips, LD. “Drug harms in the UK: a multi-criteria decision analysis”. The Lancet. 2010.
 “Drugs of Abuse”. US Department of Justice, Drug Enforcement Administration. 2017.
 Biederman and Vessel. “Perceptual Pleasure and the Brain”. American Scientist. May-June 2006. National Institute on Drug Abuse