It’s not OK to start smoking with the idea you will quit. Tobacco is so addictive that a large percentage of those who start will never stop. Many, many prominent rabbinical authorities have ruled it’s absolutely forbidden. Prominent rabbis in Israel have ruled that a smoker can’t be a kosher witness, or be a shochet, mohel or sofer. If it’s “cool” — then where there is no man, be a man, and run away from these “cool” people.
Marijuana is extremely harmful. Solid evidence links it to increased risk of all kinds of mental illnesses, even schitzophrenia. Any pot smoker will tell you it makes them “paranoid” — irrationally anxious — sometimes. It can even lead to longterm problems with panic attacks. A very dangerous drug, much worse than people think.