Updated June 08, 2015.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

The science of poisons is called toxicology. Paracelsus, a 15th century Swiss scientist and physician, created the famous dictum:

"All things are poison, and nothing is without poison; only the dose permits something not to be poisonous."

In modern medicine and biology, this is still a very important principle. It applies to the drugs we take, the chemicals we are exposed to, and in some circumstances, the food we eat.

A stark reminder of this principle is the fact that we can "poison" ourselves with water if we drink too much. Endurance athletes, like marathoners, have died after excessive water consumption, which dilutes body sodium, leading to a sometimes fatal condition called hyponatremia.

Vitamins, Minerals and Supplements

When taking supplements of any type, Paracelsus's principle should be kept in mind: More is not necessarily better, and could be much, much worse. For example, vitamin A is a powerful systemic poison at high intakes, and can also cause birth defects. Vitamin D, and minerals like selenium, zinc and iron (and others), although essential nutrients, should not be taken beyond recommended label doses because of possible toxic effects. In addition, some supplements can interact with proprietary medicines and make them less effective or even toxic. To see a summary of many vitamins, minerals and supplements, see the National Institutes of Health Dietary Supplements Fact Sheets.

Protein powders are probably safe when bought from reputable suppliers, but they may be unnecessary and costly.

When supplements are bought over-the-counter, the safety aspects of a doctor's advice and evidence-based labeling are often absent. You should be vigilant when selecting and using sports, weight training and bodybuilding supplements, especially when bought over the internet from dubious sources. This applies to some herbal medicines and naturopathic medicines also. You need to know and understand the ingredients and the safety of the product and the doses recommended. If you do not, and don't have access to medical advice, then it's wise not to proceed. Many examples exist of adverse health effects resulting from supplements that were not safe to take.

Weight Training and Bodybuilding Supplements

When deciding to take supplements, consider the following "five pillars" of selection:

1. Is the product effective? Does it give you the benefit that the sales and promotional literature says it does? Researching the active ingredients in the product is a good place to start. The NIH site (linked above), the FDA Supplements site, or a reputable medical institution site should have reliable information. The Australian Institute of Sport Supplements site is a good place to check out sports supplements for efficacy and safety.
2. Is the product safe? For many supplements and alternative health products, labeling is often not required or is less stringent than for regulated drugs and pharmaceuticals. Many of these products will not have been tested for safety under various doses and conditions. You cannot automatically assume herbal or natural products are safe. Some plant-based extracts in herbal products have been shown to be powerful liver and kidney toxins and some ayurvedic remedies have been shown to be contaminated with toxic heavy metals.
3. Does it contain a dose that is effective? Unless regulatory controls demand the appropriate labeling, you may not know if the dose recommended on the label is actually effective. The product may contain the ingredient, but unless an effective dose is applied, it may not produce the required beneficial effect. Even if the label says it has X amount, do you trust this to be accurate for this product and manufacturer? Examples: fish oil, B12, or vitamin D at very low concentrations.
4. Is the manufacturer a reputable supplier? This depends on the regulations, and the reputations of suppliers in any particular country. Make some effort to find suppliers that have documented and prominent quality assurance policies and audits. Be suspicious of cheap purchases from little-known manufacturers over the internet.
5. Do you need the product? Supplement manufacturers may promote the idea that the stresses of modern life and physical activity make you deficient in essential vitamins and minerals and that food alone will not fill this need, or that high doses of certain supplements will provide performance enhancement. Some performance supplements just do not work, and for most of us, even active professional sports people, taking in minerals like zinc and magnesium far beyond the recommended daily allowances (RDA) has never been shown to be beneficial for health or "ergogenic" for performance. Iron and zinc can be toxic in excess. Even so, taking a multivitamin has become popular to avoid deficiency states. Taking small quantities of extra vitamins and minerals in multivitamins may be worthwhile insurance, whereas taking mega doses is likely to be wasteful and perhaps even dangerous in some cases.