It only sounds like that to you because you've been confused by various mediums and authorities about what a "drug" is.
People are always telling you "don't do drugs" and they try to make it very simple because they assume you're too stupid to understand complex issues and to make value judgments of your own. (Not you personally: everyone. They underestimate the audience. This is the same approach many people take to sex education.)
The thing is, there's no difference between the drugs they sell at the drug store and illegal drugs:
All drugs affect your body.
All drugs have negative side effects.
All drugs should be treated the same: whenever you take one, you must evaluate its negative side effects against the positive effects.
For example, Tylenol can cause liver damage, so before you take Tylenol you should ask yourself: is it worth it? Is the pain bad enough this time that I should risk damaging my liver? Many times the answer is Yes, but you should still be asking yourself that question.
Parkinsons drugs are another example: they have serious side effects. At this point in time, my grandfather has decided not to take the drugs because the effects would be greater on his system than the effects of the disease are. That might change as the disease progresses: the point is, he has to continually evaluate the drugs in light of the greater context.
Illegal drugs have to be treated the same: are they worth it? The government has decided they are so not worth it that no reasonable person would choose to take them, so they've taken the decision out of our hands. But even illegal drugs have medicinal uses. There isn't much difference between heroin and morphine, but one is illegal, the other legal for restricted medical use. And more and more studies are proving the medicinal benefits of marijuana.
The point is, no drug should be taken for granted: it's dangerous to do so. That's why it's bad that the media/authorities have set things up like medicine vs. drugs -- medical drugs vs. illegal drugs -- because you end up taking the medical drugs for granted (you don't make educated, conscious choices about introducing them to your system), and you take the illegal ones for granted to (you assume they have no medical benefit).
