Drugs are not as harmful as many people think. Most drug users are perfectly healthy people who just happen to have a drug problem, just like people who smoke, or are alcoholics, or are addicted to caffeine have a drug problem. Otherwise, they are able to function properly and be good citizens. There are no recorded deaths from Marijuana in the U.S. The amount of deaths for Cocaine is 20 per 100,000 (4 if we take into account such factors as its reduced quality), and the deaths from Heroin are 400 per 100,000 (or 80 if we take these factors into account). This is compared to 150 deaths per 100,000 from Alcohol and 650 from Tobacco. Taking the numbers into account, in the U.S. there used to be 390,000 deaths per year from Tobacco, 150,000 from Alcohol and only 400 from Heroin and 200 from Cocaine (and none from Marijuana). (Source.)

Thus, legalising drugs, which would potentially make them more common, will not be a hazard to the global health status of the population.