Marijuana Use May Worsen Heart Failure and Death
This is a cautionary tale about distinguishing between correlation and causation.
Another research study illustrates the importance of separating the correlation of two things and linking a cause. The headline originated from a meeting held a few days ago by the American Heart Failure Society at its Scientific Sessions in Minneapolis, MN. It read, “Marijuana Use Increases Readmission, Mortality Risk in Heart Failure Hospitalizations.”
However, after reading the report, it should have said, “Marijuana Use is Associated with Increased Readmission, Mortality Risk in Heart Failure Hospitalizations.” It has been said thousands of times, and I will say it again,” correlation does not equal causation.”
Epidemiologists have illustrated the absurdity of equating the causation of two things in the classic example of ice cream cones and sharks. There are more shark bites in the summer because more people swim in the waters where sharks live and feed. And more people eat ice cream in the summer because it’s hot and they are outside. However, this does not mean that eating ice cream causes you to get a shart bite.
But let's return to the study cited above.
Results of the study, which included data from more than 13,000 heart failure admissions, suggested marijuana users had a 19% increased adjusted odds of readmission and 60% increased adjusted odds of death during index admission relative to their counterparts without marijuana use.
Although marijuana use is still a federal crime, 24 states have legalized its use. The rise has been significant and dramatic.
According to research published in the American Journal of Public Health in 2024, reported use of cannabis within the past year has grown to approximately 25% among the US population, with 11% reporting daily use.
Over 13,000 patient medical charts were analyzed between 2021 and 2022 for patients who were admitted emergently with heart failure. Their use of marijuana was examined to see if it correlated with heart failure readmissions and death.
Marijuana users were more likely to be younger, male, poorer, and to use tobacco. And herein lies the problem. It is known that tobacco use alone increases heart failure risks and death. The study presented last week does not detail how or if it was able to separate the use of smoking pot from cigarettes.
The headline would have been more honest if it had said this drug habit was associated with more readmissions and deaths, rather than implying that its use increases readmissions and deaths. The word 'use' does not necessarily equate with causation; however, most readers will not make this distinction.
Words matter in scientific reporting. However, had the headline been formatted differently, it might not have been a headline. And therein lies the problem.
