Understanding 'Addiction'

It can be useful to understand how a person comes to ‘need’ a drug or has a ‘dependence’. But in order to do this it is firstly important to understand how drugs effect the body, and the effects of homeostasis, tolerance and withdrawal


When a young person consumes a drug, it changes the level of neurotransmitters in the synapse. The body responds to this through biological responses that attempt to return the rate of neurotransmission to normal levels. This process is called homeostasis – a complex set of mechanisms to counteract the effects of the drug. Our body does this all the time, not just for drug use, but for normal conditions such as sweating or ‘goose bumps’ when we are too hot or cold.

For a young person who uses drugs occasionally, or for a single time, normal functioning will return quickly as the amount of drug in the body reduces and homeostatic mechanisms take action. Depending on the half-life of the drug, this may take from a few minutes to a few hours.

Half-life is the time it takes for a drug to drop from its highest level in the blood to half this level. This will depend on how long it takes the body to process and get rid of the drug.


The more often a drug is used, the more the brain becomes used to its effects, and the more established its pattern of homeostatic responses to the drug’s presence becomes. This is called neuroadaptation. A common effect of this is a young person’s increased tolerance to the drug.

When someone has developed a tolerance:

  • The same dose no longer produces the effect it used to
  • Increased doses of the drug are needed to get the same effect

Pharmacology identifies four main types of tolerance.


The presence of a drug in the blood stream causes the liver to produce what can be best described as ‘hepatic drug metabolising enzymes'. As the body becomes more efficient at this metabolisation (break down and excretion) of a drug, more is needed to gain the effect.


Receptors in the brain respond to the continued presence of a drug by either increasing in number or reducing their sensitivity to that particular drug. Whichever is the case, the effect is the same, the user will need to increase the dosage either to occupy additional receptors or, in the latter case, to deal with their reduced sensitivity.


Long experience of a drug may result in the user becoming better at dealing with its effects. Furthermore, the environment in which tolerance develops seems to have a ‘conditioning’ effect, so that tolerance is greater in the environments in which it is developed than in totally new environments. This suggest that people are at more risk of overdose in unfamiliar surroundings.

Cross tolerance 

A person who has developed a tolerance, for example, to heroin, shows increased tolerance to the whole class of drugs known as opiates.

Tolerance becomes known as dependence when the young person has a negative response when they don’t have the drug i.e. they need to take the drug just to feel ‘normal’ rather than to get the effect they first began taking it for.

Functional or Psychological

A young person can also be reliant on a substance for its function in coping with stressors that arise from difficult situations or personal issues- functional reliance. When a substance use is habitual and intensive, problems with psychological dependence can also develop.


Withdrawal refers to the physical and mental effects of reducing or stopping the use of a drug after a person has developed a tolerance to it.

When regular drug use stops, or is reduced, the altered neuroadaptation balance starts to revert to its original state. This can cause withdrawal symptoms that last until balance is restored (or until the drug is consumed again). In some circumstances, normal functions can be restored in a few days, but in others complete restoration may take a considerable time.

Withdrawal symptoms tend to be the opposite of the effects produced by the presence of the drug in the body. For example the normal effects of methamphetamines is stimulation, so the effect of sudden withdrawal is emotional depression. The normal effects of benzodiazepines is relief from anxiety, so the effect of withdrawal is increased anxiety.

How severe withdrawal is varies greatly with individuals and circumstances.

In extreme cases, and with certain drugs (alcohol and benzodiazepines) withdrawal may require medical interventions as it can be life threatening.

While a small number of people will go through severe or obvious withdrawal, a larger number will go through more subtle experience, which may include symptoms such as anxiety, confusion, insomnia and depression.


An overdose is the accidental or intentional use of a drug in an amount that is higher than is normally used (this is subject to the person's tolerance). Intoxication and overdose are most usefully thought of as existing along a continuum.

Intoxication occurs when a person consumes a quantity of a substance that exceeds their tolerance and produces some change. When this intoxication represents degrees of physical or psychological harm, it is often referred to an overdose.

The effects of overdose vary with different drugs. An overdose on CNS stimulants will cause anxiety, extreme excitability and a rising of the body’s core temperature. An overdose on CNS depressants may cause increased activity initially because of disinhibiting effects, but will lead to sedation and stupor. In extreme situations, this can progress to unconsciousness and stopping of breathing then cardiac arrest and death.

Because this is a continuum, it can be difficult to distinguish when intoxication has becomes an overdose. The best thing to do is to monitor behaviours (for safety) and conscious state and call ‘000’ if you notice any concerning symptoms.  

Further Reading