I have been teaching a college course on the effects of drugs upon the brain for over thirty years. This question always comes up. The answer depends completely on what you mean by the word "bad." For example, will smoking cigarettes or growing tobacco plants in your backyard get you arrested? No. Thus, society and its laws have already decided some of the answers to your question. What about the brain? Nicotine is far more addicting than marijuana. Which is more harmful? We now know that nicotine is neuroprotective and likely prevents Parkinson's disease. Tobacco smoke is carcinogenic but it is also a powerful metal chelator - thus, if can actually remove toxic metals from your body as you smoke. This might be useful if you work in a toxic environment. We also know that THC is an anti-inflammatory and neuroprotectant as well. THC can reduce the pain associated with multiple sclerosis and arthritis and reduce the nausea associated with some anti-cancer medications. People who smoked marijuana when they were young may be less likely to get Alzheimer's disease when they're old. Smoking marijuana is typically more irritating to the lungs than cigarettes because most smokers do not use filters. Nicotine can actually kill you at very low doses; THC has never been shown to be lethal. As you can see, answering your question is complicated by so many factors. In general, all things being equal, the epidemiological evidence is that marijuana use is associated with far fewer health problems than smoking cigarettes. Obviously, this has nothing to do with whether a drug is legal or not. If you want to know more about these topics, please read my blogs at Psychology Today.