Your body adjusts to a regular amount, and it un-adjusts when the amount stops. That is why the first use after a break is the dangerous one: the dose that was ordinary for the person you were a month ago is not ordinary for the person you are now. The World Health Organization advises everyone who has withdrawn from opioids that they are at increased risk of overdose for exactly this reason.
What tolerance is, without the jargon
Take something regularly and your body compensates. Over weeks, the same amount produces less effect, so people take more to get where they were. That is tolerance, and it is why doses climb.
Stop, and the compensation unwinds. Faster than most people expect, and much faster than it took to build. Nothing about your history protects you here, and years of experience with a substance is not a form of immunity. In one specific way it is the opposite, because experience is what tells you how much is a normal amount, and that number is now wrong.
What the sources say
Two, both plain.
The WHO's withdrawal management guidance: all opioid dependent patients who have withdrawn from opioids ["should be advised that they are at increased risk of overdose due to reduced opioid tolerance"](https://www.ncbi.nlm.nih.gov/books/NBK310652/).
The NHS: ["Detox lowers your tolerance to heroin. This means your risk of overdosing is higher if you take heroin or other drugs, such as benzodiazapines, after detoxing."](https://www.nhs.uk/live-well/addiction-support/heroin-get-help/)
Note the second half of the NHS sentence. It is not only about going back to the same drug. Taking something else on lowered tolerance carries the same problem, and combinations of things that slow breathing are the situation to be most careful about.
Any gap counts
The word "detox" makes people think this only applies to a formal programme with a keyworker attached. It does not. What matters is time without the substance, however it happened.
- A planned detox, community or residential.
- A fortnight where you just did not.
- A hospital admission.
- Time in custody.
- An illness that put you out of action for a week.
Each of those is a break, and each of them ends with the same fact: the amount you last used is not a safe guide to the amount to use now.
This is also the reason [you used again, what now](/guides/drugs/you-used-again-what-now) leads with safety rather than with feelings. The night after a clean stretch is the night this matters.
It is not only opioids
Opioids are where the risk is most acute and most documented, and they are not the only place tolerance falls.
For alcohol, the UK clinical guidelines say it directly: ["After a period of abstinence, tolerance is greatly reduced… they may even experience fatal alcohol poisoning"](https://www.gov.uk/guidance/clinical-guidelines-for-alcohol-treatment/8-harm-reduction). Somebody who drank heavily, stopped for a month and then drinks the way they used to is not repeating an old night.
The general principle is the same across substances: a break changes what your body is set up for. Treat the first time back as if it were your first time, because in the way that counts, it is.
The four habits
None of these require you to have stopped, and all of them are worth having in place before you need them.
**1. Never use alone.** This is the one that decides outcomes. Naloxone in a drawer is no use if nobody is there to give it. If you are going to use, be with somebody, or leave a door unlocked and have someone ring you at a set time.
**2. Have naloxone, and make sure someone else knows where it is.** WHO ["recommends that naloxone be made available to people likely to witness an opioid overdose, as well as training in the management of opioid overdose"](https://www.who.int/news-room/fact-sheets/detail/opioid-overdose). In the UK, drug treatment services can hand it out without a prescription, and kits can be supplied to a person at risk of overdose and to their family or friends ([GOV.UK](https://www.gov.uk/government/publications/widening-the-availability-of-naloxone/widening-the-availability-of-naloxone)). [Naloxone and keeping yourself alive](/guides/drugs/naloxone-and-keeping-yourself-alive) covers how to ask.
**3. Start much smaller than you think.** If you are going back after a gap, the old amount is the wrong number. Less, and wait.
**4. Do not mix.** Combining things that slow breathing, opioids with benzodiazepines or with alcohol, is the pattern the NHS warning names specifically.
Know what an overdose looks like
Worth learning before you need it, and worth teaching to whoever you live with.
The WHO lists the signs of an opioid overdose as ["pinpoint pupils, unconsciousness, difficulties with breathing"](https://www.who.int/news-room/fact-sheets/detail/opioid-overdose). Breathing that is slow, shallow, snoring or gurgling is not sleep.
If you see that: call 999, say what has been taken if you know, give naloxone if there is a kit, and stay. Naloxone ["is an antidote to opioids that will reverse the effects of an opioid overdose if administered in time"](https://www.who.int/news-room/fact-sheets/detail/opioid-overdose), and its effect can wear off before the opioid does, which is exactly why the ambulance still matters after somebody wakes up.
Why this page is not a warning against stopping
It would be easy to read all of this as a reason to keep going at a steady amount, and that would be the wrong conclusion.
The risk described here is specific and manageable. It attaches to one moment, the first use after a gap, and it is reduced enormously by four habits that cost nothing. What it argues for is planning the gap properly rather than not having one: naloxone in the house before you stop, somebody who knows, and something arranged for the fortnight afterwards.
If you are about to start, [what detox actually involves](/guides/drugs/what-detox-actually-involves) covers what to have in place first, and [how to stop using](/guides/drugs/how-to-stop-using-drugs) has the three first moves and the substances where stopping suddenly is itself dangerous.
FRANK is 0300 123 6600, 24 hours. We Are With You runs free local services across England and Scotland plus a webchat with trained recovery workers, and its advice where there is an immediate risk to life is to call 999. Samaritans is 116 123 at any hour.
Common questions
**How quickly does tolerance drop?** Faster than it built, and there is no safe number of days to work from. Treat any period without the substance as enough to have changed things, and go back well below your old amount if you go back at all.
**Does tolerance come back if I use once?** Not to where it was. One use after a break does not restore what you had before, which is part of why the days immediately after a return are risky rather than only the first one.
**Is this only about heroin?** The clearest documented risk is with opioids, and the NHS warning also names taking other drugs such as benzodiazepines after detoxing. UK alcohol guidelines say tolerance is greatly reduced after a period of abstinence and that fatal alcohol poisoning is possible. The principle applies broadly.
**Can I get naloxone if I am still using?** Yes. In the UK, drug treatment services can supply naloxone without a prescription, and it can be given to a person at risk of overdose as well as to their family or friends. You do not have to be in treatment or trying to stop.
**What do I do if someone will not wake up?** Call 999. Give naloxone if there is a kit. Stay with them until help arrives, and tell the ambulance crew what was taken if you know, because that is what helps them rather than what gets anyone in trouble.
Sources
- [WHO, clinical guidelines for withdrawal management](https://www.ncbi.nlm.nih.gov/books/NBK310652/), for increased overdose risk after opioid withdrawal because of reduced tolerance.
- [NHS, Heroin: get help](https://www.nhs.uk/live-well/addiction-support/heroin-get-help/), for detox lowering tolerance and raising overdose risk, including with other drugs.
- [WHO, opioid overdose fact sheet](https://www.who.int/news-room/fact-sheets/detail/opioid-overdose), for the signs of an overdose, what naloxone does, and the recommendation to make it available.
- [GOV.UK, widening the availability of naloxone](https://www.gov.uk/government/publications/widening-the-availability-of-naloxone/widening-the-availability-of-naloxone), for drug treatment services supplying naloxone without a prescription and who can be given a kit.
