The manner in which we classify a drug as dangerous, addictive, or relatively benign is based on an intricate intersection of chemistry and social norms. For example, LSD (commonly known as acid) is often seen as a drug only for the most committed of counterculture participants, while alcohol is part of many adult social interactions. However, alcohol is directly responsible for more than 2,200 deaths every year in the United States, and LSD has never been named an outright cause of death. So why aren't our drinking habits referred to as an “alcohol epidemic?”

Society often views drugs such as LSD, heroin, methamphetamine, and cocaine something “other” people use, and as a bigger threat to society than alcohol. The public sticks to this belief — valid in some instances — in large part because these drugs are illegal. They're also rarely studied in clinical research, and when they are, scientists often have different views on what makes a drug addictive or harmful. So which drugs are really as dangerous as they seem? And which substances are deceptively accepted, yet actually quite destructive?

How Is Addiction Measured?

Measuring a drug’s potential for addiction can be as complicated as assessing whether a person is addicted to a drug. That’s because when a person first becomes dependent on a substance, it isn’t always obvious. Their diagnosis may come after a breakdown of relationships, a financial crisis, or a life-changing event that opens their eyes to the way their substance use has affected them. Predicting how a drug will affect people’s addiction risk is based on a similarly varied assessment of factors: How much the drug affects their brain’s pleasure centers; the severity of withdrawal symptoms; and the drug's accessibility and cost.

There is no single factor that indicates a drug’s overall potential for harm, but rather a combination of many different factors. Some researchers place more weight on chemical effects, claiming a drug that extensively activates the brain’s dopamine system is more addictive than other substances. Consistently stimulating the dopamine system can decrease sensitivity, prompting users to take more of the substance each time to experience a high. Others say the blame is on habit and behavior, and advocate that the right way to treat addiction is to change patients’ feelings, relationships, and behavior rather than toning down the firing neurons in their brains.

Given the varied opinions, Professor David Nutt, a psychiatrist and director of neuropsychopharmacology in the Division of Brain Sciences at Imperial College, London, decided to ask a panel of addiction experts to rank which drugs they believed were most addictive. The top three most addictive drugs, according to their collective feedback, are below.

Heroin

Perhaps the most stigmatized of all illicit substances, heroin topped the list of most addictive drugs. Fear over rising rates of heroin use and addiction are well-founded; heroin hits the addiction trifecta. The opioid causes dopamine levels in the brain to increase by up to 200 percent in experimental animals, it has a cheap street value, and causes brutal withdrawal symptoms — like bones riddled with pain — capable of driving even those committed to recovery back into using. More than 8,200 people died of a heroin overdose in 2013, and heroin use has more than doubled in adults aged 18 to 24 in the last decade.

Heroin is not only majorly addictive, but it rounds out its bid for most dangerous drug with a 1:5 effective-to-lethal dose ratio. In other words, it only takes a dose five times more than that required to get high in order to cause death. By comparison, cocaine has a ratio of 1:10, and both marijuana and LSD have ratios of around 1:1,000.

Alcohol

Legal in most countries worldwide, alcohol can be insidious in terms of addiction. It’s widely accepted as a way to loosen up, and binge drinking is common across most college campuses. Despite its ubiquity, the substance is capable of increasing dopamine levels in the brain by 40-360 percent, the researchers found. And the more experiment animals drank, the higher that level rose.

Excessive alcohol use contributes to 88,000 deaths annually in the U.S., and almost 23 percent of those who use alcohol will become dependent at some point in their lives. Several experts have ranked alcohol as the most damaging drug in society, based on its harm to both users and others.

Cocaine

Though movies sometimes portray cocaine as a “rich man’s drug,” used by models and Wall Street execs, the drug’s effects are anything but glamorous. Powdered cocaine, along with its smokeable cousin, crack, work by directly interfering with the brain’s dopamine pathways, essentially making it impossible for the brain to turn dopamine signals off. This results in an “abnormal activation of the brain’s reward pathways,” wrote Eric Bowman, a lecturer in psychology and neuroscience at the University of St. Andrews, in The Conversation.

In 2009, the cocaine market was worth about $75 billion, and catered to between 14 million and 20 million people worldwide. Similarly to alcohol, about 21 percent of people who try cocaine will end up addicted at some point in their lifetime. Though cocaine is a factor in many deaths, direct overdoses are an escalating issue — the Centers For Disease Control and Prevention reported a 12 percent increase in cocaine overdose deaths from 2012 to 2013.