Why is the impact of legal drugs on health greater than illegal drugs?

More people take legal drugs, than illegal drugs.

Since only a relatively small number of people choose to break the law and use illegal drugs, the overall impact on society is limited.

In contrast, most people take a legal drug at some point in their lifetime. So legal drugs have a far greater total impact on health.

Realize however this is all just quantitative. The actual impact of any drug on an individual is going to vary greatly. A single person taking an illegal drug is going to be far more impacted by that one drug if they allow it to destroy their life than any total number of legal drugs they may take. On the other hand; that one persons downfall is but a drip in the bucket compared to the sheer volume of other people not taking illegal drugs who will only have a legal drug impacting their life.