❝ dual diagnosis refers to having both a substance misuse and mental health problem ❞

Drugs are potentially addictive if they have a psychoactive effect. People may then feel a craving or need for more of the drug. This is unlike needing a medicine such as insulin or believing in a health need to take vitamins.


Addiction is an umbrella term covering substance use and associated health and social consequences. On the 'What is addiction?' page we defined dependence as a 'psychological state which can be recognised when an individual becomes more preoccupied with substance use, spending more time drinking, smoking or taking other drugs, often at the expense of other important activities'. Why are some drugs more likely than others to lead to dependence?

The psychology

Dependence is all about what psychologists call reinforcement. What this means is how much am I likely to want to repeat a behaviour - in this case the behaviour is substance use. We have already said that, for all practical purposes, there needs to be a desirable psychoactive effect. This does not explain why drugs with similar effects differ: for example heroin is more addictive than codeine, cocaine is more addictive than caffeine, and yet each pair of drugs belong to the same drug group (opiates and stimulants).

The pharmacology

The characteristics of some drugs make reinforcement of their use stronger than for other drugs. These characteristics are not to be confused with social constructs, such as legal status, or cultural norms, such as availability of a drug. There are four important characteristics:

  • POTENCY: the stronger the drug effect the more reinforcing.
  • SPEED of ONSET: the faster the drug effect occurs the more reinforcing. The route of use and the suitability of the drug for the route will affect speed of onset.
  • DURATION: the more quickly the drug effect wears off the sooner the desire to 'top up' and if withdrawal symptoms occur the sooner the desire to use the drug to relieve the symptoms. Repeated use embeds and strengthens a memory of the desirability of the drug use.
  • PLASTICITY: when the effect of the drug is unpredictable, its use is less likely to be repeated.

You can apply these criteria to any psychoactive drug and predict its addictiveness - more correctly its dependence forming potential.

Slideshow: if you are interested in the science, take a look at the slideshow. It will take 40mins to go through all the slides so get comfortable and give yourself plenty of time to digest the learning points, or you might want to have more than one sitting to take it all in.

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is a YouTube channel where you can find video presentations on the pharmacology principles mentioned on this page.