Which is the least harmful drug? A Scientific Ranking of Psychoactive Substances

October 9, 2025 •

4 min read

Globally, tobacco, alcohol, and illicit drugs account for millions of deaths and lost years of healthy life annually [1.5.4]. The complex question of which is the least harmful drug depends on how 'harm' is defined, with scientific analyses often challenging public perceptions and legal classifications [1.3.1, 1.2.3].

Quick Summary

Determining the least harmful drug is complex. Scientific studies rank substances based on harm to the user and others, revealing that legal drugs like alcohol can be more dangerous than some illicit ones. This analysis explores these rankings.

Key Points

Harm is Multi-faceted: Drug harm is measured by physical damage to the user, dependence potential, and social harm to others.
Alcohol is Most Harmful Overall: Scientific studies consistently rank alcohol as the most harmful drug due to its massive impact on society, despite being legal [1.2.2, 1.3.7].
Psilocybin Ranks Lowest: Psilocybin (magic mushrooms) are rated as the least harmful drug overall, with low toxicity and dependence potential [1.2.7].
Legality vs. Harm: A drug's legal classification often does not align with its scientifically assessed harm level [1.3.1].
Tobacco is highly harmful: Tobacco is another legal substance that ranks as more dangerous than many illegal drugs, being a leading cause of preventable death [1.5.5, 1.3.3].
Context Matters: The harm of any drug depends on dosage, frequency of use, individual vulnerability, and setting.
Harm Reduction Saves Lives: Strategies like providing sterile syringes and naloxone are proven public health interventions that reduce drug-related deaths and disease [1.6.3].

Understanding and Defining Drug Harm

When evaluating the dangers of psychoactive substances, it's crucial to understand that 'harm' is not a single concept. Experts typically divide drug-related harm into three main categories:

Physical Harm to the User: This includes both acute effects like overdose or accidental injury while intoxicated, and chronic health problems that develop over time, such as organ damage, cancer, or infectious diseases from injection [1.4.7].
Dependence (or Addiction): This refers to the potential of a drug to cause uncontrollable, compulsive use, even when faced with negative consequences [1.4.3]. Dependence encompasses both psychological cravings and, for some substances, severe physical withdrawal symptoms [1.4.3, 1.4.5].
Harm to Others (Social Harm): This dimension considers the drug's impact on the wider community. It includes factors like economic costs (healthcare, law enforcement), family adversities, and drug-related crime [1.2.2, 1.4.6].

Significantly, a drug's legal status does not always correlate with its level of harm as determined by scientific analysis [1.3.1].

The Landmark Study on Drug Harm

A pivotal 2010 study published in The Lancet by Professor David Nutt and the Independent Scientific Committee on Drugs sought to systematically quantify these harms [1.2.2, 1.2.3]. The researchers used a multi-criteria decision analysis (MCDA) to score 20 different drugs across 16 criteria, covering harm to users and harm to others. The results provided a comprehensive ranking that challenged many preconceived notions [1.2.3].

The study concluded that alcohol was the most harmful drug overall, scoring 72 out of 100 [1.2.2, 1.3.6]. This was largely due to its immense harm to others, where it scored far higher than any other substance [1.2.4]. Following alcohol were heroin (55) and crack cocaine (54), which were deemed most harmful to the individual user [1.2.2].

The So-Called 'Least Harmful' Drugs

According to the same landmark study and subsequent analyses, some substances consistently rank at the lower end of the harm scale.

Psilocybin (Magic Mushrooms): These ranked as the least harmful drug in the 2010 UK study [1.2.7]. Psilocybin is considered to have a low toxicity and low potential for creating dependence [1.8.4, 1.8.5]. The primary risks are psychological, such as inducing fear, paranoia, or a 'bad trip', and the danger of misidentifying poisonous mushrooms in the wild [1.8.1]. In controlled settings, it is considered relatively safe [1.8.3].
LSD and Ecstasy (MDMA): These substances also ranked among the least damaging in the UK analysis [1.2.3]. Their harm scores were just a fraction of those for alcohol, heroin, or cocaine [1.2.6]. However, risks exist, including psychological distress and, for MDMA, potential negative interactions with other substances and the risk of dehydration or hyponatremia (over-consumption of water) [1.6.2].
Cannabis: Cannabis consistently ranks in the lower-to-mid range of harm [1.3.1, 1.3.5]. While its use is not without risk—particularly for adolescents, in high concentrations, or when smoked—its overall harm score is significantly lower than legal substances like alcohol and tobacco [1.7.4, 1.3.1]. Long-term use can affect memory and lung function, but studies suggest alcohol is more damaging to brain health [1.7.1, 1.7.3].

Comparing Legal Drugs: Alcohol and Tobacco

A critical finding from drug harm studies is the high level of damage caused by legal, regulated substances.

Alcohol: As noted, alcohol was ranked the most harmful drug overall in the UK study, primarily due to its massive societal impact [1.2.2, 1.3.7]. It is linked to a wide range of diseases, contributes heavily to injuries and violent crime, and carries a significant economic cost [1.7.2, 1.2.2].
Tobacco: Tobacco was also ranked as more harmful than several illegal Class A drugs, falling just behind cocaine in one analysis [1.3.3]. It is the leading cause of preventable death in the United States, killing more people annually than alcohol, illegal drugs, and car accidents combined [1.5.5].

Drug Harm Comparison Table

This table is based on the overall harm scores from the 2010 UK study by Nutt et al., which ranked 20 popular psychoactive substances [1.2.2, 1.3.6]. The score is out of a possible 100, combining harm to users and harm to others.


Rank	Drug	Overall Harm Score	Harm to User	Harm to Others
1	Alcohol	72	34	46
2	Heroin	55	34	21
3	Crack Cocaine	54	37	17
4	Crystal Meth	33	32	<10
5	Cocaine	27	11	16
6	Tobacco	26	13	13
7	Amphetamine	23	13	<10
8	Cannabis	20	10	10
9	GHB	18	13	<5
10	Benzodiazepines	15	11	<5
11	Ketamine	15	10	<5
12	Methadone	14	9	<5
13	Mephedrone	13	7	<5
14	Butane	10	10	<1
15	Khat	9	5	<5
16	Ecstasy	9	5	<5
17	Anabolic Steroids	9	6	<5
18	LSD	7	4	<5
19	Buprenorphine	6	3	<3
20	Mushrooms	5	2	<3

Source: Adapted from The Lancet, 2010. Harms to others and users are estimated from the study's charts and may not sum perfectly to the total due to weighting. [1.2.2, 1.2.4]

The Importance of Harm Reduction

Given that no drug is completely harmless, a public health approach known as 'harm reduction' aims to minimize the negative consequences of drug use without necessarily requiring complete abstinence [1.6.5, 1.6.6]. Strategies include:

Providing sterile equipment: Syringe service programs (SSPs) reduce the transmission of HIV and hepatitis [1.6.3, 1.6.4].
Overdose prevention: Distributing naloxone to reverse opioid overdoses is a life-saving measure [1.6.4].
Education and testing: Providing information on safer use practices and offering drug-checking services (like fentanyl test strips) can prevent accidental poisonings [1.6.2, 1.6.4].
Managed programs: For severe alcohol use disorder, managed alcohol programs (MAPs) provide small, regular doses of alcohol to prevent withdrawal and reduce binge drinking [1.6.2].

These evidence-based strategies improve public safety and connect people to treatment services [1.6.3].

Conclusion

Scientific evidence provides a clear, if complex, answer to the question of 'which is the least harmful drug?'. Based on comprehensive analyses of harm to both users and society, psilocybin mushrooms rank as the least harmful, followed by substances like buprenorphine and LSD [1.2.7, 1.2.2]. Perhaps the most significant insight from this research is the immense harm caused by the legal and widely used substance, alcohol, which ranks as the most damaging overall [1.3.7]. This disparity highlights that a drug's legal status is often a poor indicator of its actual risk. Understanding the relative harms of different substances is essential for developing rational public health policies, effective education, and life-saving harm reduction strategies [1.2.1].

Disclaimer: This article is for informational purposes only and does not constitute medical advice. The use of any psychoactive substance carries risks. Consult with a healthcare professional for guidance on substance use and addiction.

Authoritative Link: The Lancet [1.2.2]

Frequently Asked Questions

According to a landmark study published in The Lancet, alcohol is considered the most harmful drug overall when combining its effects on the user and on wider society [1.2.2, 1.3.6].

Scientific analysis ranks psilocybin, the active compound in magic mushrooms, as the least harmful psychoactive substance, with a very low overall harm score [1.2.7, 1.2.2].

Overall, yes. While heroin is more harmful to the individual user, alcohol's devastating impact on others (through crime, economic costs, and family issues) makes its total harm score higher than heroin's [1.2.3, 1.2.2].

Cannabis is ranked in the lower-middle range of drug harm, significantly less harmful than alcohol or tobacco but with more potential for harm than psilocybin or LSD [1.3.1, 1.2.2]. Risks are associated with smoking, adolescent use, and high-potency products [1.7.4].

Drug laws are often based on historical and cultural factors, not just scientific evidence. This is why legal substances like alcohol and tobacco are rated as more harmful than many illegal drugs like LSD, ecstasy, or psilocybin [1.3.1, 1.2.4].

The main risks of psilocybin are not from physical toxicity but from psychological distress, such as anxiety, paranoia, or having a 'bad trip.' There is also a significant danger of accidentally ingesting a poisonous look-alike mushroom if foraging in the wild [1.8.1].

Harm reduction is a public health strategy focused on minimizing the negative consequences of drug use. It includes programs like syringe exchanges to prevent disease and distributing naloxone to prevent overdose deaths, without necessarily requiring abstinence [1.6.5, 1.6.6].