I am a qualified pharmacist. I have worked in community pharmacies and hospital pharmacy for close to 30 years.
With regard to cannabis, when it is smoked regularly over a lengthy period of time it is known to induce a range of mental disorders. A well known example is called amotivational syndrome. This presents as a lack of drive, motivation and energy. The individual effected become apathetic and loses interest in in setting and striving toward life goals. They also become sedentary and inactive (physically) their speech and reaction times slow down markedly as well. These symptoms are an ideal seed-bed for the emergance of mood disorders like dysthymia or major depressive disorder.
Although there have not been many large, mainstream, scientific,, research investigations in Oz into the long-term side-effects of smoking moderate to large doses of THC on a regular basis ( recreational cannabis is illegal in Oz, so research subjects are difficult to recruit). On a personal note, I have encountered many, long-term cannabis users over the years who displayed clear symptoms of amotivational disorder.
I would bet money that the abuse of recreational cannabis could very likely afflict users with depressive- type mood disorders.
Lastly, it seems to me that many people regard cannabis as a harmless, "toy" drug, that you can just fool around with whenever you want. theres a rule of thumb in pharmacology that says if you use a drug that hits your head pretty hard like speed/amphetamines/Cocaine:, oxycodone, LSD, Valium, Ecstasy (MDMA0) strong alcohol, etc; they you are playing wit fire, and it would be a good idea to throw the shit in the bin. Cannabis is "head gear" when you smoke a goodish amount of it , it really hits your head, Right" It alters your consciousness in unusual ways because it is known to have a psychotomimetis effect as well (i.e, something similar , thought milder than what hallucinogens like "Magic Mushrooms (Psilocybin), MDMA and mescaline have.
My advise is don't fuck around with drugs like the stuff I've mentioned above (including cannabis); they're really dangerous and if you get hooked on any of them you'll wish you were dead.
Dachshund - the WONDER HOUND
DLM - Dachshund Lives Matter !!