Contrary to what most people nowadays would say, I think it shouldn't be legalized. Where I'm from, people believe that there are no adverse effects to it's use. What they don't know is that it has both physical and mental effects on the body.
MEMORY LOSS: This problem comes from the effects of cannabis on the hippocampus which is the part of the brain responsible for short term memory. The way the body works, every memory we have first starts as short term memory before being transferred to long-term storage. Thus by affecting the hippocampus, the cannabis hinders the person's ability to create new memories and learn anything new. Research shows that people who smoke cannabis tend to develop false memories. Further research showed that teenagers who smoked a lot where found to have irregularly shaped hippocampus by their twenties. It also showed that people who smoked in their younger years were bound to have memory loss problems when they got old.
DOPAMINE PRODUCTION: Dopamine is a chemical produced by neurons to send signals to other neurons. It is also produced when the body experiences something pleasurable. Thus dopamine induces happiness when produced. Cannabis usage produces extra dopamine in the brain thus resulting in the commonly sought "high" feeling. What people don't know is that the brain will then adjust it's own dopamine production in an attempt to reach its normal level again. Continued use will just make the brain keep on reducing the dopamine it produces and in the end cannabis users will have to depend on cannabis for dopamine that their bodies would have produced.
EFFECTS ON THE HEART: Cannabis smoke consists of many harmful chemicals like ammonia and hydrogen cyanide. These chemicals damage the lungs and bronchus after long time exposure. Aside from that, the part of the smoke that actually enters the blood stream causes the heart to beat as much as 50 times more than usual. this causes the heart to weaken and eventually can cause a heart attack in both old and young people, with or without any prior heart problems.
ADDICTION: Research shows that one in ten cannabis users are addicted to the substance. when considering teenagers, the ratio drops to one in six. Setting the addiction aside, withdrawal from cannabis use is also a problem as those who use the substance, whether addicts or not often experience a loss of appetite, depression and anxiety when not on the substance.
Taking all these things into consideration, I'd say its best to leave cannabis as an illegal substance and if possible, make normal cigars and cigarettes illegal too