The answer is sometimes an enigma. Two bottles may seem to have similar ingredients but one is clinically effective and the other not. For those that are not clinically effective, it can be a costly mistake.
My microbiology professor in medical school ran an experiment on probiotic supplements, culturing many different brands. Of the several brands she tested, unfortunately only three brands matched the advertized label. All the other probiotics did not meet label claims, for example, if the bottle claims 10 billion lactbaccilus accidophilus per servings, only about 4 billion were present indicating the product was not manufactured to be shelf stable. Even more shocking she found that some brands contained unwanted bacterial contaminants.
This is why I take the time to research supplements I use for my patients to ensure that they recieve the quality that they pay for and that they do not inadvertently get pesticides, heavy metals, PCBs, unwanted microbes in their products. I may not be able to tell if a certain product found on the internet is of good quality, because it is not possible to put hours of research to investigate the quality control protocols of the many companies that are out there. I am however careful to choose products in my practice i can be confident that are of the highest quality.
When you purchase supplements, ensure that they are GMP (Good Manufacturing Practice) certified. For example, Metagenics, a company I use in my practice is certified by three third party GMP organizations, one of which is an Australian agency, TGA (Therapeutic Goods Administration) who's standards are 10 times stricter than the FDA in the United States. Metagenics is the only company to achieve GMP certification from 3 leading external quality control agencies. That is not to say that there are not many other wonderful companies who manufacture products that are effacacious, environmentally friendly, and pure.
The shocking video below shows how supplements can be manufactured in people's home kitchen, and illustrates that nutritional supplements vary in quality. It is important to do your research or consult a health professional who is knowledgeable in this field. However, this article is not intended to create paranoia about supplements but to empower you to choose sources that are manufactured properly, meet label claims, and give you what you pay for. The graph (right) shows how safe nutritional supplements are compared to medications even if some were not manufactured in "Cadillac" quality facilities. In 2008, the US had 48,700% more AERs (Adverse Event Reports) for prescription drugs compared to supplements. According to the FDA, an AER is anything from a concern that a supplement isn’t working, to a serious illness that follows the consumption of said supplement. In conclusion, using a natural product is still more safe, produces less side effects, and is less invasive than prescription drugs.