First, I think you are making the wrong comparison. Your compairison should be marijuana versus tobacco and marijuana versus alcohol. Marijuana compares very favorably to both tobacco and alcohol use.
The opiates that are approved for opiate medical use are (1) synthesized opiates, meaning they are chemically destilled from the original opium, and (2) there are a number of medical studies behind those drugs. The problem with marijuana is that there have been few studies on the medical benefits of marijuana or its synthesized version THC.
I personally don't use marijuana, tobacco or alcohol. Marijuana and alcohol can make you do some stupid things -- and I'm stupid enough as it is -- and I just don't get the benefits of tobacco except for being expensive and causing long, painful deaths. However, I believe that as long as tobacco and alcohol are legal, marijuana and psychedelics, such as LSD, should be made legal. I would venture that more people are hurt in Alabama from alcohol use than are hurt in the entire United States from marijuana use. And tobacco use is the number one cause of death in the nation.
There has been the arugment that marijuana is a gateway drug to addictive illegal opiates. I believe that marijuana is a gateway drug only because it is iillegal. The same people that sell weed on the streets also sell the harder drugs. Since you make more money selling the harder drugs, it's easy to see why dealers try to get their customers hooked on addictive drugs. I believe that if marijuana was legal, then it would be no more a gateway drug than alcohol.
Moreover, you can't regulate or tax what is illegal. By legalizing marijuana, you can control what is sold. You can put a stop to a lot of the dangerous mixtures that are currently being smoked. While marijuana is probably safe for consumption in homes and apartments, we might need to have places where psychedelics are used in order to protect the users during their trips. We could do that if we made them legal.
With legalization come benefits. We remove one of the most popular drugs, weed, from the dealer. Yes, dealers will still sell weed just as bootleggers, even in places where alcohol sales are legal, still sell beer and whiskey. But far more beer and whiskey are bought from legal sources than illegal sources. So, if marijuana was legal, the vast majority of the sales would be legal sales. This would make it more difficult for dealers to get customers.
We also stop wasting resources on trying to combat a drug that is illegal only because it is not popular with the ruling elites. If members of Congress, corporate heads, political powerbrokers, etc., smoked weed like they drink alcohol, marijuana would be legal. It takes police time to arrest people for possession of pot. It takes prosecuting time to prosecute those charges. It takes a jail cell and resources that could go elsewhere to house the marijuana user. It takes a probation officer to supervise the user after he leaves jail. And, it takes money to make the probationer pee in a cup. There are dangerous drugs out there that should be illegal -- cocaine, herion, meth. Every dollar we pay to stop marijuna use is a dollar we can't use to combat those drugs.
Finally, we can tax weed. Alcohol and tobacco taxes bring in billions of dollars to local, state and federal coffers every year. Right now, the only thing that we are doing is spending money in a losing battle to stop the use of pot. We can save money and make money by making marijuana legal.