Just because something occurs naturally does not make it safe eg hemlock, digitalis.
The reason so many side effects are known about with "conventional" drugs is that the drug companies have taken the time ( and money) to test them properly. They know the optimium dosage, the lethal dose etc.
Just because no side effects are known about with a herbal remedy doesn't mean they don't exist. just means no one looked.
the advantage of prescription drugs is that the side effects are known about recorded, and the doctor can watch out for them. he can test your liver function etc. Mistakes do happen occassonally, but generally i trust in the medics.
there are a lot of people (who often wear brightly coloured clothes) who are cashing in on the "natural" remedies who know nothing of brain chemistry or drug interactons. Some natural remedies are dangerous, (for instance i was sold raspberry leaf tea when 12 weeks pregnant (causes miscarriage)), some of which are ineffective, and some of which actually work. ( but at what dose and with what long term side effects ?????))
many conventional drugs are derived from natural sources: peniciilin, digitalin, aspirin. Given the choice between taking a known dose of aspirin in convenient pill form and ripping some bark off a willow tree to make a foul tasting tea (whilst hoping that the concentration of salicylic acid in this particular tree isn't so high as to cause kidney failure) I'd plump for the "commercial" product every time.