In short: Illegal drugs carry risks to both physical and mental health, ranging from accidents and poisoning to dependence, heart problems and psychosis. In the UK, the most commonly used illegal drugs include cannabis, cocaine, ecstasy (MDMA), ketamine and nitrous oxide. Street drugs can also contain unexpected and much stronger substances, such as synthetic opioids called nitazenes, which greatly increase the risk of overdose. Confidential help is available and nobody should be afraid to seek medical care.

This guide, written by a Ready Health prescribing pharmacist, gives balanced, factual information about common illegal drugs in the UK, their effects and their health risks.

Which illegal drugs are most commonly used in the UK?

Official surveys for England and Wales consistently show cannabis as the most widely used illegal drug, followed by powder cocaine, then drugs such as ecstasy, ketamine and nitrous oxide, particularly among young adults. Drugs are classified under the Misuse of Drugs Act 1971 into Class A, B and C, with Class A carrying the heaviest penalties. Classification reflects legal status, not how safe a drug is for an individual.

Common illegal drugs: effects and risks at a glance

Drug	UK class	Common short-term effects	Key health risks
Cannabis	B	Relaxation, altered perception, slowed reactions, increased appetite and heart rate	Anxiety, paranoia, psychosis (especially high-strength forms and early use), dependence, lung problems if smoked
Synthetic cannabinoids ("spice")	B	Unpredictable, often much stronger than cannabis	Seizures, collapse, severe agitation, psychosis, dependence
Cocaine and crack cocaine	A	Alertness, confidence, raised heart rate and blood pressure	Heart attack, stroke, anxiety, paranoia, dependence; greater danger when combined with alcohol
Ecstasy (MDMA)	A	Euphoria, energy, closeness to others, raised body temperature	Overheating, dangerously low blood sodium from drinking too much water, low mood after use
Amphetamine ("speed") and methamphetamine	B / A	Energy, alertness, reduced appetite	Heart strain, anxiety, paranoia, psychosis, dependence
Ketamine	B	Detachment, distorted perception, numbness	Accidents, serious bladder and kidney damage with regular use, dependence
Nitrous oxide ("laughing gas")	C	Brief light-headedness, giggling, dizziness	Falls, loss of consciousness, nerve damage from vitamin B12 depletion with heavy use
GHB / GBL	B	Euphoria, drowsiness, loss of inhibition	Very small margin between effect and overdose, unconsciousness, severe withdrawal
Heroin and other opioids, including nitazenes	A	Euphoria, drowsiness, pain relief	Slowed or stopped breathing, overdose and death, dependence, infections from injecting
LSD and magic mushrooms	A	Hallucinations, altered sense of time and self	Frightening experiences, accidents, worsening of existing mental health problems
Street benzodiazepines (for example fake "diazepam")	C (most)	Sedation, reduced anxiety	Often contain stronger unlicensed drugs; overdose risk, especially with opioids or alcohol
Anabolic steroids	C	Increased muscle growth (non-medical use)	Heart disease, liver damage, mood changes, infertility, hormone disruption

Classes shown are those in force at the time of review. Legal status can change following government decisions.

Why are street drugs becoming more dangerous?

Contamination and mis-selling: drugs sold as one thing may contain another. UK public health bodies have issued repeated warnings about nitazenes, extremely potent synthetic opioids, found in heroin, fake prescription tablets and other drugs.
Higher strengths: the strength of cannabis, ecstasy tablets and other drugs varies widely and is often much higher than people expect.
Mixing substances: combining drugs, or drugs with alcohol or prescribed medicines, increases the risk of overdose, particularly with opioids, benzodiazepines, GHB and alcohol, which all slow breathing.

How do illegal drugs affect mental health?

Many drugs can trigger anxiety, low mood or paranoia, and some can cause psychosis. People with existing mental health conditions may find their symptoms worsen, and comedown or withdrawal can bring severe low mood. Drugs can also interact with prescribed medicines such as antidepressants and ADHD medicines, sometimes dangerously.

What should I do in an emergency?

Call 999 if someone is unconscious, struggling to breathe, having a seizure, very hot and confused, or has chest pain.
Put an unconscious person who is breathing into the recovery position and stay with them.
Tell paramedics what has been taken if you know. Their priority is treating the person, not getting anyone into trouble.
If naloxone is available and opioid overdose is suspected, use it and still call 999. Local drug services can provide naloxone and training.

Frequently asked questions

Is cannabis safe because it is natural?

No. Cannabis can cause dependence and, particularly in high-strength forms, increases the risk of psychosis. Medical cannabis products are only prescribed by specialists in limited circumstances.

Can I be honest with a doctor or pharmacist about drug use?

Yes. Healthcare professionals need accurate information to treat you safely and should respond without judgement. Information is kept confidential except in rare situations involving serious risk.

Can illegal drugs interact with my prescribed medicines?

Yes. For example, MDMA with some antidepressants can increase the risk of serotonin toxicity, and stimulants can add to the effects of ADHD medicines on the heart. Ask a pharmacist confidentially if you are unsure.

Where can I get confidential advice?

Talk to FRANK (0300 123 6600) offers free, confidential information. Your GP or a local drug and alcohol service can also help.

Key takeaways

All illegal drugs carry physical and mental health risks, and these vary from person to person.
Street drugs are increasingly contaminated with potent substances such as nitazenes.
Mixing drugs, alcohol and medicines greatly increases the risk of overdose.
Call 999 in an emergency and tell responders what was taken.
Confidential, non-judgemental support is available.

Sources and further reading

Last reviewed: October 2026 by Shamir Patel, Independent Prescribing Pharmacist, Ready Health. This article is general information and does not replace advice from your own clinician.

If you are worried about your own or someone else's drug use and would like to talk to a clinician in Standish, Wigan, book a confidential consultation with Ready Health. We can listen, review any medicines you take and help you find the right support.