There are several types of drugs, and each one affects your brain and body differently. They can be grouped by how they work: stimulants speed the body up, depressants slow it down, opioids block pain, and psychedelics change how you see things.
That grouping by mechanism is how clinicians think about substances. Understanding the category drugs fit into helps because a substance’s effects follow from how it works. It can also tell you the risks of using different drugs and the warning signs of harm and dependency.
If your own drug use is starting to worry you, or you’re trying to understand what someone in your life is going through, Castle Health can answer any questions you have.
The main types of drugs, and how they’re classified
Clinicians generally group drugs by how they act on the brain and body. A drug’s mechanism tells you more about its effects and risks than its legal status or how dangerous people think it is. The main drug types are stimulants, depressants, opioids, psychedelics, inhalants, cannabinoids, dissociatives, and empathogens.
Read more about drug addiction and how dependence develops with various types of drugs.
Stimulants
Stimulants speed up the central nervous system; they mainly work by raising dopamine, a chemical your brain uses to signal reward.
Common stimulants include cocaine, amphetamine, methamphetamine, and prescription medications such as Ritalin. They can cause:
- An increase in energy levels
- Feelings of being more alert
- More confidence
- Heart rate increase
- Various side effects such as sweating and jaw grinding.
The main risks of stimulants are cardiovascular strain and dependence, particularly with regular use. Heavy or prolonged use of cocaine or methamphetamine can also cause psychosis, including paranoia and hallucinations. There is also a risk of dependency on prescription stimulants.
Depressants
Depressants slow the central nervous system – they:
- Reduce brain activity
- Slow your breathing and heart rate
- Making you drowsy.
Alcohol is the most widely used depressant. Others include benzodiazepines such as diazepam and prescription sedatives.
Various depressants carry overdose risk. Alcohol poisoning and benzodiazepine overdose can both be fatal. The risk rises further when depressants are combined; drinking alcohol while taking benzodiazepines can slow breathing to a dangerous level.
Regular use of depressants can also lead to physical dependence. Withdrawal from alcohol and benzodiazepines in particular can be medically serious, so it’s safest to stop with medical help.
Opioids
Opioids are sometimes grouped with depressants because they also slow the body down, but they work through a different pathway.
Opioids act on receptors in the brain that control both pain and breathing. The same action that makes them strong painkillers can also slow breathing until it stops. This is the main risk of opioid overdose, which – according to the European Drug Report 2026 – causes more drug-related deaths in Europe than any other drug type.
If someone is hard to wake or breathing slowly after taking opioids or depressants, call 112 straight away.
Common opioids include heroin, fentanyl, and prescription painkillers such as morphine and codeine. Even when prescribed for pain, opioids can become difficult to stop without professional help. Our page on recognising the signs of opioid dependency goes into more detail.
Psychedelics
Psychedelics alter perception and mood by acting on serotonin receptors in the brain.
Serotonin is a key chemical in how we perceive things and how we feel. Classic psychedelics such as LSD, psilocybin (magic mushrooms), DMT, and mescaline work by activating one type of serotonin receptor, and this produces the psychedelic effects.
The effects vary widely with the dose and the person. Risks include intense psychological distress and paranoia during a trip, and psychosis in people who are vulnerable to it.
Inhalants
Inhalants are defined by how they’re taken rather than by a shared mechanism. They’re volatile substances found in household products, including glue, aerosols, solvents, paint thinner, and nitrous oxide. When inhaled, they produce a rapid, short-lived intoxication with effects similar to alcohol.
Research into inhaled hydrocarbon-associated sudden collapse has shown that inhaling hydrocarbons – the chemicals in many of these products – can cause sudden cardiac or respiratory arrest. People also collapse due to physical exertion or stress during or immediately after use.
If someone collapses, call 112 or local emergency services and follow the instructions until help comes. If you know CPR, start it straight away.
Cannabinoids
Cannabinoids are the active chemical compounds found in cannabis that change how our mind and body feel.
The main psychoactive compound in cannabis is THC. It binds to cannabinoid receptors in the brain that play a role in mood, memory, and appetite. Effects can include:
- Relaxation
- A changed sense of time
- Increased appetite
- Anxiety
Research from King’s College London found that people who first started using cannabis to self-medicate for pain, anxiety, or depression might report higher levels of paranoia. Cannabis also carries a psychosis risk, particularly in younger users and those who use it regularly.
There are different reasons and mechanisms to how cannabis use develops into dependency.
Dissociatives
Dissociatives are drugs that block ‘NMDA’ receptors in the brain – these normally help carry signals between the brain and the senses.
When those signals are disrupted, the result is a feeling of detachment from your body and your surroundings. Common dissociatives include ketamine and DXM (found in some cough medicines).
Guidance from the British Association of Urological Surgeons warns that long-term ketamine use can damage the bladder and make urination painful. In serious cases, the damage can spread to the kidneys and cause kidney failure. Stopping can help the bladder heal, especially if you stop early, but treatment won’t work while ketamine use continues.
Read more about ketamine addiction.
Empathogens
Empathogens produce strong feelings of emotional connection and empathy. According to the Australian Drug Foundation, drugs like MDMA (ecstasy) do this by causing the brain to release serotonin and dopamine.
MDMA raises body temperature, and in hot, crowded environments like nightclubs, this can lead to heat stroke.
Taking MDMA with other drugs that raise serotonin, including some antidepressants, can also cause serotonin syndrome. This is when serotonin builds up to dangerous levels in the body. Many people also experience low mood in the days after use, as serotonin levels recover. This is often called a comedown.
If someone overheats or becomes confused after taking MDMA, call 112.
“I help people understand the patterns that keep them stuck, so they can move from survival into a more grounded, connected and fulfilling way of living.”
When to seek help with drug use
There’s no single moment when drug use becomes a problem. But there are signs you can look for.
- Using more of a substance than you planned, or more often than you intended
- Needing the substance to feel normal or to get through the day
- Wanting to stop or cut down but finding it difficult
- The substance affecting your work or relationships
- Worsening physical or mental health
These signs don’t amount to a diagnosis, but they’re a good reason to look more closely at your drug use. Read more about how drugs affect your mental health.
How to get help with drug use
Castle Health offers both outpatient and residential treatment. Residential programmes run at Castle Craig Hospital in Scotland and Smarmore Castle in Ireland. You don’t need a referral, and you’re welcome to ask questions first.
You can also speak to your GP about drug use and anything that is concerning you. They can give you a full check-up and refer you to specialist services if needed.
How does dependency develop?
If you’d like to understand how dependency develops, get in touch with our team who can help you.
If someone in your life needs help with drug use
If you’re worried about someone else, you’ve probably noticed changes you can’t quite explain. They might be pulling away from people or reacting differently from the person you know.
It’s normal to feel unsure about raising it at all, but you don’t need to know which substance is involved to start a conversation. Describe what you’ve seen, and leave the explanations for later.
Castle Health can talk you through what you’re seeing and help you plan your next step. If the person in your life does go on to get treatment, Castle Health’s family support programme offers therapy for families during that process.
Frequently asked questions about types of drugs
What types of drugs are illegal?
There’s no single answer across Europe. Each country sets its own drug laws, and a substance that’s illegal in one country may be regulated differently in another. For an overview, the EUDA classification of controlled drugs explains how EU member states classify substances under UN conventions.
Which types of drugs are considered controlled substances?
A controlled substance is one where the supply, production, and use are regulated by law. That doesn’t always mean it’s illegal. Many controlled substances, including opioid painkillers and benzodiazepines, are prescribed by doctors for legitimate medical use. “Controlled” means the substance is monitored because of its potential for harm or dependence.
Can a drug belong to more than one category?
Yes, different kinds of drugs can fit in more than one category. Cannabis is a common example. Depending on the dose and the strain, it can produce effects associated with depressants, psychedelics, or both. Categories are useful as a framework, but there is overlap.
Is there a difference between a drug’s legal classification and how addictive it is?
Yes, because legal classification reflects a broader view of harm, including social harm and the potential for misuse, as well as how addictive a substance is. Alcohol, for example, is legal in most countries but carries a serious risk of dependence.
