Chronic use of cannabis has been known to alter lung tissue similar to how the way tissue that precedes cancer in tobacco smokers is altered. Though there are no studies that definitively state that chronically smoking cannabis will cause cancer. Repeated smoking of cannabis has, however, been linked heavily to chronic coughing, increased sputum production in the lungs and chronic bronchitis. It has also been suggested that repeatedly deeply inhaling and holding of the smoke also causes pneumothorax, more commonly referred to as a collapsed lung. Also, the risk among cannabis smokers for getting Tuberculosis is greatly increased due to inhalation practices and sharing water pipes or hotboxing.

Basically, just because cannabis isn't necessarily linked to cancer, per se, doesn't mean it isn't completely harmless. There are diseases and increased likelihood of disease when chronically using it.

Another more apt side effect of Cannabis usage is the increased likelihood of developing an addiction to it and spending increasing amounts of ones time spacing out or high rather than doing more productive things or tending to one's responsibilities like one's job, or parental duties.