The nicotine in cigarettes is addictive, and is in a class of chemicals called alkaloids. This class also includes cocaine, morphine and even caffeine. They all act by short-circuiting the brain’s pleasure systems, by releasing chemical messengers in the brain that make you feel good. The brain eventually starts to crave this stimulation, and that’s part of the reason cigarettes are so addictive. Nicotine itself has also been linked to problems with the circulation and other physiological effects.
But that’s not that’s in cigarettes. There are lots of other chemicals in there, most of which are pretty bad for you. The US health dept has a list of over 500 chemical additives in cigarettes, that when burned can produce even more compounds, many of which are carcinogenic – cancer forming. Overall, cigarette smoking can cause a dramatic increase in the risk of developing cancer, heart and lung disease
A lot of similar problems apply with cannabis – cannabis smoke can potentially cause mutations in your cells, which could again lead to increased risk of cancer. Cannabis also has psychological effects. Long-term cannabis use can lead to increased risk of developing schizophrenia, depression and mental health disorders.
A bigger problem with cannabis is that it’s often smoked in combination with tobacco. Getting the worst of both worlds. But for the psychological risks AS WELL as the physiological ones, I’m going to say cannabis.
Andy is correct in that when you smoke cannabis it is usually together with tobacco, so you do get the worse of both worlds. However, most people who smoke will smoke between 20-40 cigarettes a day, where as most cannabis smokers will smoke no where near that amount.
Therefore i would say that cigarettes are a lot worse for you in the real world.