When you go to the pharmacy, you may pick up medications dispensed by a pharmacist which required a prescription, over-the-counter medications which did not require a prescription, or dietary/herbal supplements.
The major difference is how each of these is classified and how they are regulated. Prescription and over-the-counter medications are classified as drugs whereas supplements are classified as foods.
In order for something to be advertised or marketed for the purpose of treating, diagnosing, preventing, or curing diseases, it must be classified as a drug. Prescription and over the counter drugs are regulated by the U.S. Food and Drug Administration (FDA) for safety and effectiveness. Before a drug can be sold it must obtain approval from the FDA to be tested in volunteers (usually thousands) over a number of years and then prove to the FDA that the drug is safe and effective, that the labeling and packaging is accurate and appropriate, and that the product has been manufactured in a way that ensures the strength, quality and purity of the drug.
Dietary and herbal supplements, classified as foods, cannot be advertised for the purpose of treating, diagnosing, preventing, or curing diseases and the FDA does not have the authority to review dietary supplement products for safety and effectiveness before they are marketed. The Dietary Supplement Health and Education Act is a law requiring manufacturers of dietary supplements to produce products which are safe, but they do not need to prove that they are safe or effective before they are sold. In other words, you must take the manufacturers word for it as to the contents, quality, and potency. That is not to say that these products are not what they say or that they are unsafe, just that their contents are not routinely analyzed by the FDA.
So how can you educate yourself about the safety of the supplements you use or are thinking about using? A good place to start is by visiting the website of the National Institutes of Health Office of Dietary Supplements. Here you can find dietary supplement fact sheets which contain summaries of the health effects, safety, recommended amounts, and medication interactions of specific vitamins, minerals, herbs, and other supplements. You may also want to contact the manufacturer of the specific supplement you are interested in to request more information.
The FDA offers some basic tips for safer use of dietary supplements which can be found here. A few of these include checking with your doctor, pharmacists, or healthcare provider before using supplements as they may interact with other medications, may be toxic if consumed in excessive amounts, or may cause unwanted effects during surgeries.
The most important thing to keep in mind when considering herbal and dietary supplements is your own safety. Not all supplements are safe, just as not all medications are safe. The difference is that you know exactly what you are taking when you use medications. The same cannot be said for supplements because the manufacturers are not required to prove the safety, quality, contents, or purity of the products they sell.