This is a very good question. And the answer is multi-faceted.
First and foremost, in the case that you are taking other prescribed medications, you should consult a physician before beginning to take a supplement. If you have any concern it is better to be safe, even though no serious side effects have been shown for most high-quality dietary supplements.
Secondly, many supplements on the market lack third party verification. Meaning, they either don't have sufficient evidence to support their claims or the studies that have been performed are inconclusive. Raspberry Keytones are a good example of this: They are touted as the latest and greatest weight loss supplements, yet studies show them to be only as effective as a placebo.
Last of all, and most importantly, some supplement companies don't source from reputable suppliers and thus either contain low quality active ingredients or none at all even though they are stated on the product labels. This is something to watch out for.
Make sure that the companies you source from have GMP certified manufacturing processes. These ensure the factory and ingredients are held to high standards. This is a great indication of a trustworthy company and quality product.
Look for a company that guarantees their products with a 100% refund policy. Businesses that don't do this should be avoided at all costs. If the company doesn't trust that they will work, why should you?
At HulaProducts, we only the highest quality ingredients and all of our supplements are GMP certified and made in the USA. We hope you enjoy our quality line of products and we look forward to hearing from you!