You're being suckered about a certain central science question, all the time. Even my best-educated non-scientist friends haven't a clue. And who can blame them? You can't turn on the TV, open a newspaper, or go into a supermarket without being bombarded with misinformation on this topic. It also happens to be my pet peeve.
Question: In general, are natural substances (isolated from living sources) any better, safer, or healthier than man-made substances?
Answer: no. Not only does everyone get this wrong, but even when I explain it, they still have trouble believing it. It is so ingrained in our heads that natural equals healthy that even questioning this dogma is difficult for most people.
The main reason (aside from marketing) that people find this to be both hard to believe and counterintuitive is that there is a fundamental misunderstanding out there that somehow man-made substances are "chemicals" while natural substances aren't. I know -- most people, when pushed, will recite what they think educated people are supposed to say: "everything is a chemical." But they either don't really believe it or think that the "chemicals" from nature are somehow different.
There is nothing magical about the way our bodies work. They take in nutrients, break them down, then use the resulting substances to provide energy and synthesize vital enzymes and other proteins, as well as fats and carbohydrates. The job of enzymes is to promote chemical reactions that make all life possible. All life processes are governed by chemical reactions. The field of science dealing with these reactions is biochemistry. Examples: the liberation of energy from fat and carbohydrates is called oxidative phosphorylation; digestion of protein is called peptide hydrolysis; conversion of glucose to starch is called glycosidation. There are thousands. Many of the chemical reactions in our bodies can be duplicated in a lab.
So, given that we are essentially "walking chemical reactors," the obvious question is, do our bodies handle natural chemicals any differently than they do man-made chemicals? While this is a rather large generalization, the answer is: not really. In order to understand why, one needs to know a little about how chemical substances are processed in our bodies.
It's All the Same to the Liver
After being swallowed, substances (foods, minerals, drugs, etc.) enter the gut, where many of them pass into the bloodstream. The first stop is the liver, our main metabolic organ. The liver determines the fate of most compounds -- whether something is broken down, built up, or left intact, excreted or stored. But the liver doesn't know (or care) what the origin of a particular substance is. It only "looks" at the chemical structure of each particular molecule and "decides" how to process it using a variety of metabolic enzymes that it contains. All chemicals have their own unique metabolic fate, no matter where they come from. So, while it is simple for chemists to determine whether something is naturally-occurring or man-made, forming generalizations about how either class will actually behave in a living organism is nearly impossible.
Plant-derived chemicals get an especially easy ride, but this sentiment is especially flawed. Most people have the impression that something coming from a plant is probably safe, but this is nonsense. In fact, many plants and herbs make chemicals called alkaloids, some of which are among the most toxic substances known. These alkaloids are not found in plants by accident. They are typically there as a defense against being eaten, so the fact that plants produce so many poisonous chemicals is certainly logical.
The real harm here is that most people wouldn't walk through a hardware store and think "Hmm...perhaps this week I'll try eating some Drano and paint thinner," but they wouldn't have any problem doing the equivalent in a health food store. And if you believe that just because something says "natural" it is risk free, then you're being taken.
Toxicity Chart Surprises
Below is a table that compares the toxicity of a number of representative substances. Keep in mind that these data are derived from LD50 values from mice and rats, not humans. LD50 means the dose of a particular chemical that kills half the test animals. Also, keep in mind that these data represent a single oral dose for each chemical and thus do not address issues of chronic exposure or carcinogenicity. The least-toxic substance (aspartame) was assigned an arbitrary value of 1 and others compared to this. Thus, the higher numbers indicate more toxicity.
How to interpret the table: Small differences between chemicals are essentially meaningless. For example, one should not conclude that aspirin (rel. tox. 4) is twice as safe as mothballs (rel. tox 2); rather that neither is acutely toxic in mice. Larger differences (i.e., Ecstacy  vs. Tylenol ) are meaningful. Ecstasy is clearly more toxic than Tylenol. Some of the numbers may seem absurd (caffeine seems to be more toxic than nicotine). This is most likely a function of species differences or experimental error. You'll see more-toxic substances can easily come from more-natural sources. There is no "nature good, man-made bad" rule.
Take a look. See what is really poisonous.
Dr. Bloom, who resides in Nyack, NY, is an organic chemist. He has worked in the pharmaceutical industry for about twenty years. See also: ACSH's Holiday Dinner Menu of natural carcinogens.