Related to
Early Years

Written by Sue Younger, MA(Hons), MCW (Hons), DipTchg, DipProfEth, Trustee

While many people can use cannabis and not seem to suffer ill-effects directly, the younger, the longer, and the more often people use cannabis, the more likely they are to suffer from a range of harms later in life. It may not be at all clear to them, or others, that their use of cannabis has contributed to their difficulties.

The harms of cannabis use, unlike some of the negative effects of alcohol, are not immediately obvious. Although cannabis reduces concentration, lowers motivation and impairs judgement, users often report experiencing relaxing, enjoyable effects in the short term. This can easily overshadow the long-term health risks associated with even moderate use.

Two groups who are particularly vulnerable are (1) the unborn babies of pregnant women, and (2) adolescents. Adults are also at risk though, as we shall see, as the use of cannabis at any age increases an individual’s risk of poor life outcomes.

(1) Using cannabis while pregnant

Babies’ brains develop more than they ever will again while in their mother’s uterus. Most of the brain cells needed for life are there when the baby is born, an astonishing 86 billion or so tiny neurons. Later development involves ‘connecting up’, as well as ‘pruning’ these connections, but the formation of brain cells is thought to largely happen during pregnancy.

Substance use of any kind in pregnancy is potentially harmful. It can get in the way of healthy brain development. The effects may last a lifetime for the baby.

It’s true that, because parents who are using cannabis may also be drinking alcohol and/or smoking tobacco and perhaps also experiencing stress, it is difficult for researchers to ‘tease out’ which of these causes what harm. But in several large studies, where babies are followed up for many years, the potential harm of cannabis is clear. The risks of cannabis are still there, even when researchers control for other factors in the lives of those who use cannabis.

At birth. I te whanautanga.

Babies exposed to cannabis before birth are:

  • More likely to be small. In turn, lower birth-weight babies are at greater risk of illness and difficulty. This includes death during infancy, as well as ongoing health and learning problems that can still affect them as teenagers and adults.
  • More likely to be admitted to a neo-natal intensive care unit, which is for very sick babies.1

At pre-school. Te kōhanga.

Even if all seems well at birth, some of the effects of pre-birth exposure to cannabis can show up later.

  • Three and four year olds whose mother used cannabis while she was hapū (pregnant) are more likely to have problems with memory and concentration.2 This can negatively affect their learning during this crucial pre-school time, when most children learn an amazing amount that sets them up for life.

Starting school. Te tīmata i te kura.

Five and six year olds whose mother used cannabis while she was pregnant are more at risk of:

  • Hyperactivity. Hyperactive children are constantly active and sometimes disruptive and hōhā (troublesome).
  • Having poor concentration, making it harder to learn.
  • Impulsivity. Manawa rere. Their ability to regulate their own behaviour may not be as good as those around them.
  • Depression. Just like adults, children can feel a low mood (pōuri).
  • Problems with sleeping and eating.
  • Problems with relationships.
  • Difficulty planning and making good choices.3

Teenage years and early adulthood. Taiohitanga.

As adolescents and young adults, those whose mother used cannabis while she was pregnant:

  • Are at risk of using their brain a bit differently from others when thinking about choices and making decisions. Some of them seem to need to use more areas of their brain.4
  • Are more likely to smoke tobacco, use alcohol, and use cannabis while they are young, and to start earlier than others. (It’s important to remember that the earlier they start drinking, the more at risk they are of developing problems with alcohol.) While it’s easy to think that they are just ‘copying’ their parents who ‘role model’ using drugs, it has been found that teenagers whose mothers used cannabis during pregnancy are more likely to use cannabis, whether or not their parents are still doing so. So, the cannabis exposure during pregnancy is itself associated with a risk of using drugs in adolescence.

Summary – Cannabis use while pregnant

For the baby, there is no safe time or safe amount of cannabis use during pregnancy. When mothers use cannabis, their baby is more likely to be small and/or sick. Long-term, their baby is at higher risk of having problems with concentration, memory and making poor choices later in life.

The ‘heavier’ the cannabis use, the higher the risks for the baby.

On a positive note, those who have been exposed to drugs in utero can have their risks lowered again. Stable, nurturing and safe environments after birth can make a positive difference for babies affected by prenatal drug use, although clearly it is preferable that they not be exposed in the first place.

One thing to note – Planned vs Unplanned Pregnancy

There is good evidence that 40% of pregnancies in NZ are unplanned.5 This means that women of childbearing age who use cannabis are potentially putting babies at risk in the early part of pregnancy, because they may not realise they are pregnant. The same is true for alcohol and other substance use.

(2) Using cannabis as a young person. Tarukino me ngā taiohi.

Adolescent brains are different from adult brains.

Brains change significantly throughout the teenage years, and into the early twenties. It is a time of important development, physically, socially, emotionally, intellectually and behaviourally. Drug use during these years can impact in harmful ways on each of these areas of development. It has the potential to disrupt a successful transition into adulthood.

There is good evidence that using cannabis during their teenage years and in their early twenties puts young people at greater risk of long-term effects than similar use by adults, because their brains are still developing and changing.6

Rangatahi who use cannabis are more likely to continue using it once they are adults, which in turn puts them at higher risk of poorer life outcomes in a diverse range of areas (see list below of poor outcomes of cannabis use once aged 30-35.)

And just as with alcohol, the younger a person is when they try cannabis, the higher their chances of becoming “dependent” on the drug. Dependency on cannabis brings with it a risk of a range of life problems, and places people at higher risk of mental and physical health problems. About 17% of people who start using cannabis in their adolescent years will become addicted.7

While many adolescents can ‘experiment’ with drugs and not go on to have serious problems in adulthood, the safest option for adolescents is to refrain from all drug use while their brain is still developing.8 For those who have already started, reducing or stopping altogether can lessen the risks.

(3) Adult use of cannabis. Ngā pakeke me te tarukino.

Many studies have shown associations between ongoing cannabis use and poor outcomes in people’s lives.

In New Zealand, a Christchurch study has shown that, by the time people are in their early 30s, those who regularly smoke cannabis:

  • have a greater likelihood of being dependent on cannabis, other illicit drugs and/or alcohol,
  • are more likely to suffer from mental health problems, including psychosis, mania and suicide9,
  • have lower achievement in education,
  • earn markedly lower income,
  • have poorer physical health,
  • have less likelihood of being in a relationship and are less likely to have dependent children,
  • have increased odds of relationship violence,
  • have an increased risk of arrest and conviction.10

The bigger picture.

The evidence is growing that, especially in the long-term, those who use cannabis are taking risks with their health and wellbeing. The following quote is from a group of researchers in the Christchurch longitudinal study, who have followed a large group of babies since they were born in the late 1970s.

“…it is now clear that those who adopt a lifestyle characterised by frequent or chronic cannabis use are at the risk of long-term harms to health, social and economic wellbeing in later life.”11

One NZ study found that nearly 80% of young people have used cannabis on at least one occasion by the time they are 25, while 13% are involved in heavy use and meet the criteria for “dependence”.12

For those who are not ‘dependent’, whether their use of cannabis will cause them harm or not depends on many factors. Each individual is different but we know that some things, such as experiencing or witnessing violence and living in poverty make babies, children and adults more vulnerable to harm. These things are “risk factors”. Other things, such as being part of a loving family and a ‘connected’ community, make them more likely to turn out well. These are ‘protective factors’. Each baby, child and adult has their own unique set of risk and protective factors.

In this way of looking at individuals, cannabis functions as a “risk” factor. As with most things in life, those who are already vulnerable due to other ‘risk factors’ in their lives are even more likely to suffer harm from cannabis use. For example , those at genetic risk of certain psychiatric illnesses increase their risk of illness when they also smoke cannabis.13

And, in turn, using cannabis adds to a vulnerable person’s load of “risk factors”, making them more vulnerable to harm from other factors in their lives, such as poverty or illness.

Sadly, those who are vulnerable are also the most likely to use cannabis. In New Zealand, those who use cannabis most frequently, and most heavily, are more likely to have the most disadvantaged family, childhood and individual characteristics.14


While people report positive feelings, excitement, euphoria and comfort from the use of drugs in the short-term, the long-term effects can be harmful, as we have seen. This is especially true for babies exposed to cannabis before birth, adolescents and those with, or at risk of, poor mental health.

Glossary of Māori Terms

Hapū – pregnant
I te whanautanga- at birth
Manawa rere – impulsivity
ngā pakeke me te tarukino – adults and cannabis
Rangatahi – young people
Te kākano – the seed
Te kōhanga – the nest or nursery
te tīmata I te kura – starting school
taiohitanga – teen years
Tarukino me ngā taiohi – cannabis and young people

An afterthought. For discussion.

We know that cannabis has the potential to cause harm and puts people at higher risk, yet it is widely used in New Zealand. Are there other things, such as economic stability, educational success, social support, strong whanau and strong communities, that may provide the ‘comfort’ and ‘highs’ of drug use without putting babies, young people and adults at risk?

  1. Volkow et al., 2017
  2. Wu et al., 2011
  3. Fried, 2002; Wu et al., 2011
  4. Smith et al., 2016
  5. Morton et al., 2010
  6. Lisdahl et al., 2013
  7. Hall et al., 2009, cited by Volkow et al., 2014
  8. Winters & Arria, 2011
  9. Memedovich et al., 2018
  10. Boden et al., 2019
  11. Boden et al., 2019, p. 10
  12. Boden et al., 2006
  13. Gage et al., 2016
  14. Boden et al., 2019


Boden, J. M., Dhakal, B., Foulds, J. A., & Horwood, L. J. (2019). Life‐course trajectories of cannabis use: a latent class analysis of a New Zealand birth cohort. Addiction.

Boden, J. M., Fergusson, D. M., & Horwood, L. J. (2006). Illicit drug use and dependence in a New Zealand birth cohort. Australian and New Zealand Journal of Psychiatry, 40, 105-113.

Fried, P. A. (2002). Conceptual issues in behavioral teratology and their aplication in determining long-term sequelae of prenatal marijuana exposure. Journal of Child Psycholgy and Psychiatry Studies, 43(1), 81-102.

Gage, S. H., Hickman, M., & Zammit, S. (2016). Association between cannabis and psychosis: Epidemiological evidence. Biological Psychiatry, 79(7), 549-556.

Lisdahl, K. M., Gilbart, E. R., Wright, N. E., & Shollenbarger, S. (2013). Dare to delay? The impacts of adolescent alcohol and marijuana use onset on cognition, brain structure, and function. Frontiers in Psychiatry, 4, 53.

Memedovich, K. A., Dowsett, L. E., Spackman, E., Noseworthy, T., & Clement, F. (2018). The adverse health effects and harms related to marijuana use: an overview review. CMAJ Open, 6(3), E339-E346.

Morton, S. M., Atatoa-Carr, P., Bandara, D., Grant, C. C., Ivory, V. C., Kingi, T., . . . Pryor, J. (2010). Growing Up in New Zealand: A longitudinal study of New Zealand children and their families. Report 1: Before we are born. Auckland, NZ: Growing Up in New Zealand.

Smith, A. M., Miouszewski, O., Hatchard, T., Byron-Alhasson, A., Fall, C., & Fried, P. A. (2016). Prenatal marijuana exposure impacts executive functioning into young adulthood: An fMRI study. Neurotoxicology and Teratology, 58, 53-59.

Volkow, N. D., Baler, R. D., Compton, W. M., & Weiss, S. R. (2014). Adverse health effects of marijuana use. New England Journal of Medicine, 370(23), 2219-2227.

Volkow, N. D., Compton, W. M., & Wargo, E. M. (2017). The risks of marijuana use during pregnancy. JAMA, 317(2), 129-130.

Winters, K. C., & Arria, A. (2011). Adolescent brain development and drugs. Prevention Research, 18(2), 21-24.

Wu, C. S., Jew, C. P., & Lu, H. C. (2011). Lasting impacts of prenatal cannabis exposure and the role of endogenous cannabinoids in the developing brain. Future Neurology, 6(4), 459-480.

This article was first published by Brainwave Trust Aotearoa in May 2020