For example, tobacco on it's own is relatively harmless - the side effects of smoking are mostly caused by tar and nicotine. Alcohol is relatively harmless even at a pint a day level. Alcohol is only a danger when people exceed limits, or drive whilst under the influence.
Weed does however cause paranoia issues, which can easily spiral due to it being a dependence drug to make people feel better.
I reckon that Medical Marijuana, in the form of pills, at a low dosage, should be legal. However there should be plenty of restrictions on it to stop recreational use and false prescriptions.