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Overdose Basics

What is an Overdose?

An overdose occurs when a person has more of a drug (or combination of drugs) in their body than the body can cope with. All drugs can cause an overdose, including medication prescribed by a doctor. It is important to know the right amount and the right time to take your medication. It is also vital to know what drugs should not be mixed (see more about polydrug use below), and to seek help if you feel you are not in control of your drug use. 

Drug overdoses can be both fatal and non-fatal. Fatal overdoses – or overdose deaths – can be either intentional, unintentional (also known as accidental), or of undetermined intent. 

There are many similar words and terms that are used in reference to overdose and overdose deaths. Some examples are drug toxicity or drug poisoning (or drug toxicity deaths or drug poisoning deaths). These terms refer to the adverse effects in the body – including death – that can result from drug use, whether those drugs that are illicit or licit and taken at either therapeutic or non-therapeutic doses

Overdoses can look different. But there are a number of signs and symptoms that show someone has overdosed, and these differ with the type of drug used. Learn more about how an overdose might appear depending on the drug type and how you can respond below. 

Download this full set of fact sheets about how to respond to an overdose by drug type, which can be printed and shared.”

Get the Fact Sheets

Know the signs of overdose when it comes to different drug types and learn how you can respond to overdose.

Depressants and Opioids

A depressant is a drug that slows the vital activities of the body including breathing and the heart rate. Depressants may also be known as sedatives. Opioids (such as heroin and pharmaceutical opioids like Endone), benzodiazepines (such as Xanax or Valium), barbiturates and alcohol all slow the central nervous system to produce a calming effect. 

These substances are often prescribed to relieve pain, help you sleep, or in the case of alcohol, used recreationally. However, when taken in excessive amounts or in combination, they can depress normal functions such as breathing and heart rate until breathing and the heart eventually stop, resulting in death.

Download the Opioids Fact Sheet for more information.

Download the Depressants Fact Sheet for more information.


Fentanyl is a synthetic opioid analgesic which is commonly used for medical purposes to manage acute, severe, or chronic pain. Though it is used safely and in a legal manner in healthcare settings throughout the world, fentanyl and fentanyl analogs (i.e., substances that appear pharmacologically or chemically similar to fentanyl) are also found within the illicit drug market.  

Drug contamination with fentanyl and its analogs is a major global public health concern due to the significant risk of opioid-related overdose and other harms. Fentanyl is highly potent – 50-100 times more potent than morphine[i] – and many people may be unaware that they are consuming fentanyl.  

The use of fentanyl analogs has increased in recent years[ii]. Carfentanil is the most common type of fentanyl analog trafficked in the United States and it is approximately 10,000 times more potent than morphine[iii].

To reduce the risk of overdose and other harms from fentanyl and its analogs, learn more about drug testing or drug checking services or the availability of fentanyl test strips in your jurisdiction. Learn more about drug contamination or drug adulteration further below.


Generally people do not automatically think of alcohol when they think of overdose, but alcohol is a depressant and it is possible to overdose on it. Acute alcohol poisoning, which is usually a result of binge drinking, is an example.

If you drink a large amount of alcohol quickly the level of alcohol in your bloodstream (blood alcohol concentration, or BAC) can become dangerously high. This can stop your body from working properly. In extreme cases, alcohol poisoning could stop you breathing, stop your heart or cause you to choke on your own vomit.

Download the Alcohol Fact Sheet for more information.


It is possible to overdose on amphetamines such as speed and ice. Amphetamine overdose increases the risk of heart attack, stroke, seizure or drug-induced psychotic episodes.

Download the Stimulants Fact Sheet for more information.

Permanent Brain Damage and Overdose

All drug misuse can lead to brain injury. Hypoxic brain injury, which is caused by a lack of oxygen to the brain, is an under-reported consequence of overdose. This can lead to coma, seizures and death. The long-term consequences of hypoxia depend on how long the brain is without an adequate supply of oxygen. The longer a person is not breathing, the more damage is being done to their brain.

A brain injury can result in mild to severe impairment of:

  • Movement, balance and co-ordination.
  • Senses such as hearing or vision.
  • Spoken and written communication.
  • Thinking, concentration and memory.
  • In severe cases, brain injuries from overdoses can leave people in a vegetative state.

Polydrug Use or Polysubstance Use

‘Polydrug use’, also known as ‘polysubstance use’, refers to the use of more than one drug or drug type by an individual at the same time or sequentially. Polydrug use includes the use of both illicit and licit drugs, including alcohol, tobacco, new psychoactive substances, prescription medication, and more. 

Polydrug use can be either intentional or unintentional. Unintentional polydrug use may occur when a person unknowingly consumes adulterated or contaminated drugs. For example, a person might believe they are using cocaine, but it may also contain another substance or substances, such as the synthetic opioid fentanyl. Unintentional polydrug use may also occur when a person uses a drug with a long half-life that is still in their system, and they later use another substance (this may be the next day). For more on tolerance and half-life, see below.

The prevalence of polydrug use – whether intentional or not – has been increasing in many parts of the world. For example, in North America, polydrug use of stimulants (such as cocaine and methamphetamine) in combination with synthetic opioids (particularly fentanyl) has increased to such an extent that it has been referred to as the “fourth wave” of the opioid overdose epidemic[iv]. 

It is important to know that polydrug use can increase the risks associated with drug use – including the risk of overdose – due to the interactions between different substances. These interactions can amplify the effects of a drug, mask the effects of one or more drugs, or produce unpredictable side effects. For fuller understanding of risks, it is also important to understand more about drug tolerance and half-life (see more below). 

Tolerance and Half-life

If someone uses a drug regularly they develop a tolerance to it. This means they need to use more of the drug to get the same effect. 

Just as a person can develop tolerance, they may also lose it if they haven’t used a drug for a while. When people take their usual amount of drugs after a break from using, it could be too much for their body to cope with and this may lead to an overdose. This is why periods of abstinence from drug use, such as after release from prison and after a period of detoxification and/or rehabilitation, are risk-factors for overdose.

‘Half-life’ refers to the time it takes for a drug to drop to half the strength of its original dose. Some drugs, such as some benzodiazepines, have a long half-life. A person who has taken drugs may still have enough in their system the next day to overdose if they use more. 

For example, the half-life of Diazepam (Valium) is about 24 hours. So, if you took 20 milligrams yesterday you would still have approximately 10 milligrams of Diazepam in your system today. If you were then to use an opioid like morphine or heroin, you would have an increased risk of overdose as you would be using the opioid in addition to 10 milligrams of Diazepam.

Adulterated or Contaminated Drug Supply 

Drug adulteration or drug contamination refers to the presence of substances in a drug other than those that are expected. Adulterants may be deliberately added to drug supply for many reasons, such as increasing bulk or potency in order to improve profit margins.  

Adulterants can include legal and readily available substances such as chemicals, caffeine, paracetamol, and sugars[v]. Adulterants can also include substances such as synthetic benzodiazepines and cathinones, synthetic opioids like fentanyl, and xylazine.   

It is important to emphasize that an adulterated or contaminated drug supply means that the risk of overdose increases in the community. Though illicit drugs are more commonly adulterated with benign substances[vi], more harmful adulterants (such as unregulated synthetic benzodiazepines and cathinones, fentanyl, nitazenes and other novel opioids, and xylazine) are increasingly detected in drug supplies around the world.  

To manage the health risks of drug adulteration or contamination, learn more about local drug testing or drug checking services in your jurisdiction. 

First Aid for Drug Overdose

A range of signs and symptoms can occur when a person overdoses, and everyone responds differently. Signs and symptoms depend on a variety of factors including which drug is taken, the amount taken and the person’s state of health at the time.

If you can’t get a response from someone, do not assume they are asleep. Sometimes it can take hours for someone who has overdosed to die.

An overdose is a medical emergency that requires immediate medical attention. Always call an ambulance if you suspect someone has overdosed.

When to Call an Ambulance

People are often reluctant to call an ambulance for fear of police involvement or concern about the cost of a call-out. However, you should always call an ambulance if you suspect someone is at risk of overdose. In many places, the police will only attend if there is a fatality or other circumstances warranting police attendance, such as a threat to the ambulance crew.

Seeking emergency help isn’t just for when someone is unconscious. You should also seek emergency help when someone is:

  • Having a seizure.
  • Experiencing severe headache.
  • Experiencing chest pain.
  • Experiencing breathing difficulties.
  • Extremely paranoid, agitated and/or confused.

It is not necessary for someone to have all of these signs or symptoms for them to be overdosing. Exhibiting one or two could still mean they are in trouble and need emergency help.

Don’t Ignore Snoring and Gurgling

Snoring and gurgling can indicate that a person is having trouble breathing.

With substance use, especially substances that slow down the systems of the body (eg: benzodiazepines, opioids, GHB), snoring may indicate a serious and potentially life-threatening obstruction of the airway.

In the context of substance use, snoring is not something that should be seen as “normal”. Don’t let someone “sleep it off” if they are snoring; this may be a sign of significant and life-threatening emergency. You should attempt to wake them immediately.

If they do wake up, then the snoring (airway obstruction) will resolve. If they do not wake up, call emergency services and ask for an ambulance. Follow the instructions from the operator (you will probably be asked to roll the snoring person onto their side and open their mouth to maximise air flow).

Naloxone or Narcan® 

Naloxone (also known as Narcan®) is the drug used by paramedics to revive people who have had an opioid overdose. Naloxone can cause withdrawal symptoms and a powerful urge to take more of the drug that led to the overdose. Anyone who has been revived using naloxone should understand the risks involved in taking more drugs afterwards. Taking more drugs after being administered naloxone can result in a second overdose. This is because the half-life of naloxone (60-90 minutes) is considerably less than heroin and morphine. Drugs like methadone and sustained-release opioids such as oxycodone (brand name OxyContin) release doses of the drug slowly over 12 hours or more. So, the effect of naloxone will wear off long before those drugs have left a person’s system.

Learn more about Naloxone on this page or by downloading this info sheet.

[i] World Health Organisation (2021). Opioid overdose factsheet
[ii] Tennyson, Kristin M., Charles S. Ray, and Kevin T. Maas (2021). Fentanyl and fentanyl analogues: federal trends and trafficking patterns. Washington: United States Sentencing Commission. 
[iii] Tennyson, Kristin M., Charles S. Ray, and Kevin T. Maas (2021). Fentanyl and fentanyl analogues: federal trends and trafficking patterns. Washington: United States Sentencing Commission. 
[iv] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8154745/
[v] Cole C, Jones L, McVeigh J, Kicman A, Syed Q, Bellis M. Adulterants in illicit drugs: a review of empirical evidence. Drug Test Anal. 2011 Feb;3(2):89-96. doi: 10.1002/dta.220. Epub 2010 Dec 29. PMID: 21322119.
[vi] Cole, Claire and Jones, Lisa and McVeigh, Jim and Kicman, Andrew and Syed, Qutub and Bellis, Mark A (2010) Cut: a guide to adulterants, bulking agents and other contaminants found in illegal drugs. Liverpool: Faculty of Health and Applied Social Sciences, Liverpool John Moores University. 

Overdose Prevention

Learn more about evidence-based overdose prevention strategies, the signs of overdose and how to respond, some of the main risks of overdose, and more about dependence, addiction and treatment.

Overdose affects everyone. From grieving families to spontaneous first responders, the impacts of overdose are far-reaching and fall indiscriminately.

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