Mental and behavioural disorders due to the use of opioids, cannabinoids, sedatives and hypnotics, stimulants, and hallucinogens: acute intoxication, harmful use, dependence, withdrawal syndrome, and psychotic disorders. Treatment

01Definition and overview

Drug use disorders share the categories used for alcohol: intoxication, harmful use, dependence, withdrawal, and the psychotic and other disorders a drug induces. The criteria for dependence are the same for every drug.

The fourth character gives the syndrome, as for alcohol: .0 intoxication, .1 harmful use, .2 dependence, .3 withdrawal, .4 withdrawal with delirium, .5 psychotic disorder. ICD-11 lists each class separately (6C41 to 6C4G) and adds classes for synthetic cannabinoids, synthetic cathinones, MDMA and dissociative drugs. DSM-5-TR calls dependence a substance use disorder, graded mild, moderate or severe.

A drug-induced psychotic disorder in ICD-10 starts during or within two weeks of use, lasts more than 48 hours and resolves, at least partly, within a month and fully within six months. Psychosis that persists with abstinence, or began before the drug use, points to a primary psychotic disorder.

02Etiology and pathophysiology

Every drug of dependence raises dopamine in the nucleus accumbens, directly, as stimulants do by blocking or reversing the dopamine transporter, or indirectly, as opioids, cannabinoids and sedatives do by releasing dopamine neurons from GABA inhibition. With repeated use, cues linked to the drug come to trigger strong wanting, ordinary rewards lose their pull, stress systems drive use to relieve withdrawal and dysphoria, and prefrontal control weakens. This model of addiction as a learned, chronic brain disorder is a simplification: environment, choice and social circumstances shape every step.

• Genes: dependence on most drugs is about 40 to 60 per cent heritable, largely through shared genes for addiction and impulsivity.
• Early life: childhood adversity and trauma, early first use, conduct disorder and ADHD.
• Social: availability, price, peers, deprivation, homelessness and prison.
• Mental illness: anxiety, depression, PTSD, personality disorders and psychosis, in both directions.

03Clinical features

Opioids

Heroin withdrawal begins 6 to 12 hours after the last dose, peaks at 2 to 3 days and fades within a week; methadone withdrawal begins later and lasts weeks. Its severity is scored with a scale such as the Clinical Opiate Withdrawal Scale. Injecting brings its own harms: HIV, hepatitis B and C, abscesses, cellulitis, deep vein thrombosis, endocarditis and septicaemia.

Nitazenes, synthetic opioids often stronger than fentanyl, now appear in heroin, counterfeit tablets and other drugs. They caused outbreaks of deaths in the Baltic states, where they were linked to about half of drug-induced deaths in Estonia and Latvia in 2024.

Cannabis and psychosis

Cannabis is the most used illicit drug in Europe. About 1 in 10 people who use cannabis become dependent, about 1 in 6 of those who start in their teens and a quarter to a half of daily users. Its potency has risen: resin now often contains more than 20 per cent THC and herbal cannabis 10 to 15 per cent, with little of the possibly protective CBD. In a study across European cities, daily use raised the odds of a psychotic disorder about threefold and daily use of high-potency cannabis about fivefold. Where such cannabis was common it may have accounted for about 3 in 10 new cases of psychosis in London and half in Amsterdam. In Denmark, up to 3 in 10 cases of schizophrenia in men aged 21 to 30 may be linked to cannabis use disorder. The association is strongest with early, daily, high-potency use and a family history, and the causal share is estimated, not proven.

Cannabinoid hyperemesis syndrome, bouts of vomiting and abdominal pain eased by hot showers, affects long-term heavy users and stops only with abstinence. Synthetic cannabinoids, once sold as "spice", are now also used to adulterate cannabis resin, herbal cannabis and vapes in Europe, so users may take them without knowing.

Sedatives

Benzodiazepine dependence often begins with a prescription for anxiety or insomnia, or comes from illicit tablets, some of them counterfeit and containing potent designer benzodiazepines. Withdrawal starts within a day of stopping a short-acting drug but can be delayed for up to three weeks after a long-acting one. Pregabalin and gabapentin are misused, especially with opioids, and became controlled drugs in the UK in 2019. GHB and GBL withdrawal can begin within hours and progress quickly to delirium; it is managed in hospital.

Stimulants

Cocaine is the second most used illicit drug in Europe and a growing cause of hospital presentations and deaths. Taken with alcohol it forms cocaethylene, which is more toxic to the heart. Stimulant psychosis brings persecutory delusions, hallucinations, including the feeling of insects under the skin, and repetitive behaviour; it usually settles within days to weeks of abstinence, though methamphetamine psychosis can persist.

04Diagnosis and management

Assessment

The history covers each drug: amount, route, frequency, last use, injecting and sharing, overdoses, withdrawal, treatment so far, alcohol, mental and physical health, children, housing, money and the law. The examination looks for intoxication or withdrawal, pupils, injection sites and signs of infection or liver disease. A urine drug screen confirms recent use, but immunoassays miss many synthetic drugs, including fentanyl, nitazenes and synthetic cannabinoids, and give false positives. Everyone at risk is tested for HIV and hepatitis B and C and offered hepatitis B vaccination.

Opioid overdose

1. 1

   Airway and breathing

   Open the airway and ventilate with a bag and mask and oxygen; slow, shallow breathing is the danger.
2. 2

   Naloxone

   400 micrograms intramuscularly or intravenously, repeated every two to three minutes. In a dependent patient, smaller intravenous doses titrated to breathing avoid sudden withdrawal.
3. 3

   Observe

   Naloxone lasts about an hour. Patients are watched for several hours, longer after methadone or nitazenes, and a naloxone infusion is used if sedation returns.
4. 4

   Prevent the next

   Offer treatment and a take-home naloxone kit, nasal or intramuscular, to the patient and their family or friends.

The risk of overdose is highest after a loss of tolerance, on leaving prison, hospital, residential treatment or substitution treatment, and when opioids are combined with alcohol, benzodiazepines or gabapentinoids or taken alone. The WHO recommends naloxone for people likely to witness an overdose.

Treating opioid dependence

Substitution treatment cuts the death rate by more than half, as well as drug use, injecting, HIV and hepatitis C transmission and crime. The risk of death is highest in the first four weeks of methadone treatment and the four weeks after leaving it. Early doses are often taken under supervision, at a pharmacy for example, and treatment is long term. Psychosocial support, keyworking, contingency management and help with housing, work and family go with it.

Treating other drug use

1. 1

   Benzodiazepines

   Switch to a diazepam equivalent and reduce by about a tenth of the dose every one to two weeks, slower at the end, over months; support and CBT for insomnia and anxiety. Do not stop abruptly.
2. 2

   Cannabis

   Motivational and cognitive behavioural therapy and contingency management; no drug is licensed. Withdrawal is treated by symptoms. Those with psychosis, or at risk, are advised to stop.
3. 3

   Stimulants

   Contingency management has the best evidence, with CBT; no drug is licensed. Agitation, hypertension and seizures are treated with benzodiazepines, and hyperthermia with cooling.
4. 4

   Drug-induced psychosis

   Treat agitation with a benzodiazepine, then an antipsychotic if psychosis persists. Follow up, because about a quarter of substance-induced psychoses, and a third after cannabis, later turn into schizophrenia.

These equivalents are approximate and vary between tables; tapers are adjusted to the patient. Cocaine chest pain is treated with benzodiazepines, aspirin and nitrates; a beta-blocker alone is avoided, because it leaves alpha receptors unopposed. MDMA toxicity needs cooling and care with fluids, as hyponatraemia is common.

Harm reduction

Harm reduction keeps people alive and well while they still use drugs, and makes later treatment possible:

• needle and syringe programmes and safer injecting advice;
• substitution treatment, take-home naloxone and overdose education;
• testing and treatment for HIV and hepatitis C, now curable in weeks;
• drug checking, which warns of nitazenes, synthetic cannabinoids and high-strength tablets;
• supervised drug consumption rooms, in several EU countries and, since 2025, in Glasgow.

Laws differ across Europe. Portugal decriminalised personal possession of all drugs in 2001, and Malta, Luxembourg and Germany have allowed limited adult cannabis use or home growing since 2021, while most countries keep criminal penalties.

05Epidemiology

of EU adults aged 15 to 64, about 25 million people, used cannabis in the last year

8.7per centof EU adults aged 15 to 64, about 25 million people, used cannabis in the last year

in the EU in 2024, the reported minimum

7,600drug-induced deathsin the EU in 2024, the reported minimum

of those deaths involved opioids; about a quarter involved cocaine

66per centof those deaths involved opioids; about a quarter involved cocaine

Cannabis is by far the most used illicit drug in Europe, followed by cocaine, MDMA and amphetamines; problem opioid use is concentrated in an ageing group of long-term users. Most deaths involve several drugs and occur in men. In 2025 the EU early warning system detected 50 new psychoactive substances for the first time, among about 1,050 now monitored, many of them synthetic opioids, cannabinoids and cathinones. Synthetic cannabinoids have caused outbreaks of poisoning and death, including in prisons, and the misuse of nitrous oxide has led to cases of B12-related spinal cord damage. Use of most drugs is highest in young adults, while dependence is commoner in men and with deprivation, homelessness, prison and mental illness.

06Prognosis

Opioid dependence is a chronic relapsing disorder: regular users die at about 15 times the expected rate, from overdose, infections, liver disease, trauma and suicide. Long-term substitution treatment lowers this risk, and over decades many people stop using, though often after several attempts. Most people who use cannabis stop or cut down by their 30s, and dependence often remits. Benzodiazepine tapers usually succeed with time and support, though a minority have symptoms for months. Stimulant dependence relapses often, and contingency management improves the odds. After a drug-induced psychosis, the outlook depends on stopping the drug and on whether a primary psychotic or mood disorder emerges.