Tobacco is, for many, a legal part of life. Cannabis is, for many, an illegal part of life. Both are fairly easy to acquire in New Zealand: though I must be an outlier as the first time I smelt the green weed was at a Youth Bible Camp when I was 15. One of the issues in working out which is worse for your physical health is that often both are smoked together.

But Tobacco is worse for your physical health. From the Dunedin Multidisciplinary study: please note that more Kiwis, followed from 1972 to date, have smoked cannabis than tobacco. Perhaps regulation works better than prohibition.

The 1037 study participants were 51.6% male (n = 535). Of these, 484 had ever used tobacco daily and 675 had ever used cannabis. Cannabis use was associated with poorer periodontal health at age 38 years and within-individual decline in periodontal health from ages 26 to 38 years. For example, cannabis joint-years from ages 18 to 38 years was associated with poorer periodontal health at age 38 years, even after controlling for tobacco pack-years (? = 0.12; 95% CI, 0.05-0.18; P < .001). Additionally, cannabis joint-years from ages 26 to 38 years was associated with poorer periodontal health at age 38 years, even after accounting for periodontal health at age 26 years and tobacco pack-years (? = 0.10; 95% CI, 0.05-0.16; P <.001) However, cannabis use was unrelated to other physical health problems. Unlike cannabis use, tobacco use was associated with worse lung function, systemic inflammation, and metabolic health at age 38 years, as well as within-individual decline in health from ages 26 to 38 years.

Now, before you light up, some caveats. The first is that the street potency of joints and the components in them have changed. We are seeing more “skunk” than “Cabbage”, and more people smoking “Bud” than “leaf”. Our clinical suspicion that the amount of THC matters for psychosis was confirmed when synthetic cannaboloids were marketed — inicially legally– in NZ, and there was an excess number of psychotic breaks noted within the health system.

Overall, the potency of illicit cannabis plant material has consistently increased over time since 1995 from ~4% in 1995 to ~12% in 2014. The cannabidiol content has decreased on average from ~.28% in 2001 to < .15% in 2014, resulting in a change in the ratio of ?9-tetrahydrocannabinol to cannabidiol from 14 times in 1995 to ~80 times in 2014.

The second problem is that cannabis is associated with psychosis. Fairly strongly. Again from the Dunedin Multidisciplinary Study.

Multiple linear regression analyses showed that cannabis users by age 15 and by age 18 had more schizophrenia symptoms than controls at age 26 (table). These results remained significant after psychotic symptoms at age 11 were controlled for. The effect was stronger with earlier use.

Logistic regression analyses showed that people who used cannabis by age 15 were four times as likely to have a diagnosis of schizophreniform disorder at age 26 than controls. After psychotic symptoms at age 11 were controlled for, the risk for adult schizophreniform disorder remained higher among those who used cannabis at age 15; however, this risk was reduced by 31% and was no longer significant.

Cannabis use by age 15 did not predict depressive outcomes at age 26. Use of other drugs in adolescence did not predict schizophrenia outcomes over and above the effect of cannabis use.

Now, Psychosis is bad for you. It shortens your life, increases your risk of cardiovascular death (heart attacks and strokes) and suicide. Cannabis abuse has been associated with heart attacks and strokes. Which are not diseases of early mid life, but later life.

The answer is this: both of these weeds are bad for you. If you have to burn it, don’t inhale it.