FR
Back to the articles

The law of effect

A psychoactive substance user will feel certain effects. If that same person uses the same substance at another time, in another setting, he or she may feel very different effects. And for good reason: the effects felt when someone uses a drug depend on factors other than just the substance itself. This is called the Law of Effect.

There are three factors at play

Who is the person?

  • What is the person’s age and gender?
  • What is the person’s height and weight?
  • Is he or she accustomed to using drugs?
  • Is the person in good physical health?
  • What is his or her state of mind?

Let us take an example: Three drinks of the same type of alcohol could have a very different effect on an athletic 30-year-old than on an inactive 55-year-old. And they definitely could have a different effect on a 17-year-old in a state of depression.

What is the drug in question?

  • How much of the substance was taken?
  • How fast was it taken?
  • How was it administered?
  • How pure was the drug?
  • Was it combined with any other substances?

Let us take an example: Equal quantities of cannabis can have different effects depending on whether it is eaten or smoked. The effects can also be very different if the cannabis is combined with alcohol or another substance.

In what setting was the drug consumed?

  • Where is the person doing the drug?
  • Is he or she alone or with someone?
  • What is the person’s relationship with the people he or she is with?
  • At what time of day is the person doing the drug?
  • Does he or she feel safe?

Let us take an example: The same hallucinogenic substance can produce different effects in a regular user who is among trusted friends than in a person using drugs alone, possibly for the first time.

The law of effect combines these three factors

The effect of any substance taken depends on a combination of the three factors discussed above. This combination of factors makes it almost impossible to reliably anticipate the effect a substance could produce. Therefore, past experience can make for huge differences not only from one person to the next using the same drug, but also from one instance to the next for the same person.

Law of effect


Sources : Ministère de la Santé et des Services sociaux / Action toxicomanie / Unité d’information et d’action en toxicomanie

Read more articles