Here's the thing; no one is regulating the supplement industry. And it is indeed an industry, a big one at that. Does that mean that you can't trust anything out there? No. It means that you need to be very careful about which supplements you buy. Just because something is expensive, doesn't mean it is good. However, if something is very inexpensive (compared to other similar products) there is a pretty decent chance that it hasn't been through stringent quality control. Why? Because it costs companies much more to test each batch of raw material that comes to their plants, and even more to test the final products.
So how do you know what is ok and what isn't? As a physician, I work closely with a few companies that have stringent controls. But if you are just buying something at the store, don't take the sales person's word for it (they are usually trained by the companies directly), and don't take the word of the product label either, do your own research. Here is what I recommend.
Call the company and ask the following questions regarding supplement "X".
1. Do you manufacture it? If no, you need to speak directly to the manufacturer. Some companies just label supplements produced by another company.
2. Do you test all raw materials that come to your plant for ingredient, contamination, and potency (if applicable?)
3. Do you test the product for quality post manufacture? If yes, is it random testing, or batch testing? And, is it done in house or by an independent testing company. Fair warning here, I did some consulting for a mass market brand where they would 'shop' the product having a batch tested 3 or 4 times before someone came back with the answer they wanted.
4. If I read you the batch number of the supplement "X" in my hand, can you send me a certificate of analysis. If you don't feel 100% confident with their answers, go to another brand.
Here is a link about a lawsuit being brought against a number of high profile companies that were found to have PCBs in their fish oil. And as if that is not bad enough, many of them had statements on the labels stating that the product was "screened for PCBs" or "treated to remove PCBs."
One online resource, the subscription site Consumer Lab does do random spot checks of many supplements. What they are trying to do is very important but the potential problem here is that just because one batch looks good doesn't mean the manufacturer won't change their raw materials supplier at any time, or even that the raw materials supplier is providing consistency in product.
The take home? My advice is to only by supplements from companies that have stringent quality control of their product and manufacturing from start to finish.
