There is a huge difference between natural and synthetic drugs. A natural drug is something likely to be plant form all natural and a synthetic drug is a drug that is made or mixed with chemicals.
Here is a list of natural drugs:
Here is a list of synthetic drugs:
As you can see there is a difference. The Natural drugs are much less harmful than those that are synthetic. When you play around with a substance and mix it with all sorts of chemicals of course it is going to be dangerous and harmful, more so than that that is natural.
I have listed cannabis as a natural drug, however this is debatable as by the time it reaches you it is more than likely to be sprayed with god knows what and tampered with by the dealers. This is not to say that cannabis in its normal form is not natural.
Synthetic drugs have been invented to get round the law but they are far more dangerous than the natural drug, they can be highly addictive and cause you to have nasty, trippy experience.