The definition of ‘tolerance to drugs’ is that your body has adapted to a substance, so the same amount no longer has the same effect. It can also happen with alcohol and prescribed medicines, because your body and brain adjust to their constant presence.
Tolerance is a response of the body, and it can happen even with medicines taken exactly as prescribed. Tolerance doesn’t mean that a person has dependence or addiction. That depends on how much control they have over substance use.
But a rising tolerance can sometimes be a stepping stone to dependence, and a falling tolerance can make a drug more dangerous. Knowing what tolerance is can help you understand what you or someone in your life is experiencing.
I contacted Castle Health [formerly CATCH Recovery] seeking guidance and options to support a relative who was having a difficult time both with addiction and other mental health concerns. The team were incredibly kind and informed, they took the time to go through all the options with me and helped us plan how to approach the matter. Fortunately the person has now been receiving care from [CATCH] for several months and is doing really well. A big thank you to the wonderful team! I would highly recommend this service.
What is the definition of tolerance to drugs?
The simplest drug tolerance definition is: needing bigger doses to keep having the same response. Response means what the drug does for you, like easing pain or helping you relax. With alcohol, that means a person needs more drinks to feel the same effect.
The World Health Organization (WHO) has a definition of drug tolerance that covers what changes after repeated use, and it keeps this separate from how strongly someone reacts to a first dose. Tolerance can also appear with prescribed medicines taken for a long time, such as strong opioid painkillers.
How does a person develop a tolerance to a drug?
When a person develops a tolerance to a drug, it starts with repeated use. How much someone uses, and how often, affects how much tolerance builds. With alcohol, tolerance usually follows days of drinking, whether daily or on and off.
According to WHO, tolerance also varies from person to person, partly because of genes. For each effect of a drug, tolerance builds at its own speed, so a person can get used to one effect and still feel another. With opioids, for example, tolerance to slowed breathing can build before tolerance to drowsiness.
Types of tolerance to drugs
Tolerance to drugs comes in three main types: physical, behavioural, and acute – each works differently. There is also reverse tolerance, where an effect gets stronger with repeated use, though this happens only with some effects of some drugs.
Physical (pharmacological) tolerance
Physical tolerance happens in two main ways. In the first, your body clears the drug faster, so less of it reaches your brain. This is called ‘metabolic tolerance’ or ‘dispositional tolerance’. With alcohol, the body gets better at breaking it down.
In the second, your brain cells respond less to the same amount because of changes in the parts a drug attaches to. The body can also start processes that push the other way and cancel out part of the drug’s effect.
Behavioural tolerance
Behavioural tolerance – or acquired tolerance – can depend on your habits and your surroundings. If you usually drink or use in the same place, the body can adjust to its effects there, so you feel less of the effect than you would somewhere new.
Acute tolerance – the Mellanby effect
Acute tolerance describes tolerance building within a single session of drinking. It is also known as the Mellanby effect, though it may reflect how quickly blood alcohol rises and falls instead of tolerance itself.
In 26 mostly small studies, people generally rated themselves as less drunk when their blood alcohol was falling than when it was rising, even at the same alcohol level. Their driving-simulator and self-control results were generally worse while it was falling.
What’s the difference between tolerance vs addiction?
Tolerance is about how your body responds to a substance, while addiction is about control.
The European Union Drugs Agency (EUDA) describes addiction as a disorder that weakens a person’s control over using substances. Control here means being unable to cut down or stop when you want to, even when it’s causing you harm.
Doctors see addiction as a health condition, which can be referred to as the disease model of addiction.
Tolerance can come with addiction, and it can also happen without it, so tolerance on its own can’t tell you whether someone has an addiction.
Tolerance and dependence are closely linked but not the same thing
Find out more about when tolerance tips into addiction.
How are tolerance and dependence related?
Tolerance can lead to physical or emotional dependence.
With dependence, your body has adapted, so it reacts when you stop taking the drug. Doctors call that reaction withdrawal.
Stopping suddenly can bring on dangerous withdrawal symptoms with alcohol and some medicines:
- The EUDA notes that withdrawal from benzodiazepines, a type of sedative, can be life-threatening.
- An opioid shouldn’t be stopped suddenly either, because it can bring on withdrawal symptoms which can cause a dangerous relapse.
Stopping is generally safer with a doctor’s help. In a medically supervised detox, a doctor guides you through withdrawal. If you stop using drugs and withdrawal symptoms feel severe, get medical help straight away.
Our guide to understanding dependence goes deeper.
Why increasing drug tolerance matters
As tolerance rises, a person needs more of a substance to get the same effect. With alcohol, tolerance is a recognised risk factor for developing alcohol use disorder. Tolerance can also fall, because it fades when you stop using – so the amount you took before can be more than your body can safely manage.
This is a particular risk with opioids. After detox, or after leaving drug-free treatment, tolerance to opioids drops sharply, so the risk of overdose is high if use starts again.
Opioids taken with alcohol or benzodiazepines are also a key overdose risk, and a medicine called naloxone can reverse the potentially fatal effect of an opioid overdose.
If you think you or someone else has taken too much of a drug, call 999 or 112 straight away.
Many people have a relapse during recovery. Castle Health offers continuing care after residential treatment, including online, to help prevent a relapse and to support someone if one happens. People leaving Castle Craig can also join a peer-support service to help them through difficult times in recovery where relapse risks are high.
When might increasing tolerance be a sign of something more?
Doctors assessing dependence look for more than tolerance or daily use. Increasing tolerance can point to something more when other signs come with it.
- A person finds it hard to cut down or stop, even when they want to.
- A person puts using before other parts of life, like work or family.
- A person keeps using a substance, even when it harms their health or relationships.
- A person has strong urges or cravings to use.
If you’re not sure what these mean, the team at Castle Health can talk them through with you. They’ll ask how much and how often you use, and how it fits around the rest of your life.
Where to find support and further information
Tolerance is one part of how dependence and addiction work.
For the wider picture, start with a fuller guide to understanding addiction.
If you’re wondering what happens when someone stops using, read about a medically supervised detox.
If someone you know has a high tolerance to drugs or alcohol
If someone you know needs more alcohol, or more of a drug, than they used to, you might not be sure what you’re seeing, or whether to say anything.
When you’re ready to talk, it can help to start with what you’ve noticed and ask how they’re doing. Give them time to answer, because it can take a while to find the words.
If they’d rather not talk yet, you can come back to it another day. There’s more on how to have that conversation in our family resources.
Whether or not it turns out to be an addiction, someone’s drinking or drug use can affect the whole family, and families often need support too.
At Castle Craig, family therapy is part of residential treatment. Family members are also welcome at our online workshops about addiction, with or without family therapy.
We’ve helped many people turn their lives around
Frequently asked questions about tolerance to drugs
How does the body develop a tolerance to prescribed medications?
It happens the same way as with any other drug. Repeated doses bring a gradual loss of response. Opioids Aware says the right dose of any medicine is the lowest one that gives a noticeable benefit. Check with your prescriber before you change a dose.
How can you recognise the signs of increasing tolerance to pain relief medication?
The main sign is a dose that no longer works as well. Many medicines, including opioid painkillers, can become less effective as the body gets used to the effects. If your dose no longer seems enough, talk to your healthcare team. Coming off an opioid suddenly can bring on withdrawal so don’t alter your dose on your own.
