Many people do not know that many supplements sold in stores are contaminated with toxins, chemicals, or unhealthy microbes leaving you worse off then what you started. Also, they are often at such low doses that you need to take an entire bottle to reach a therapeutic dose. High grade neutricuetical companies will better quality control with entire laboratories dedicated to ensuring pure product that is contaminant free, including heavy metals. Many of these supplements are only available in specialty stores or through your naturopahtic doctor.
Here is a video worth watching:
The Fifth Estate - Vitamins and Supplements: Magic Pills
See your naturopathic doctor to determine which supplements are safe for you.