There was a study in early 2015 that looked exclusively at American retailers and the herbal preparations on their shelves; it found that nearly none of the samples tested contained what they claimed to contain. Lots of non-herbal ingredients and random plants, but not the herb being sold. It was reported around Feb. 3 of this year. Here's one link:
https://www.washingtonpost.com/news/...-fake-herbals/
The study was reported widely, so there may be better links than that one.
Herbal remedies aren't scrutinized for anything other than drastic toxicity by the FDA, because all the manufacturers use the disclaimer that they aren't being sold as medicines. The FDA isn't allowed to do more than check whether they'll kill you. There's NO oversight of these products - they could contain NONE of the advertised herb, or 1/10 of the dose, or every bottle could contain a different dose up to and including very toxic. People have died taking these things (remember Ephedra). There's no protection because they aren't legally medicines. Anyone who buys these things needs to know that.
If it comes from your own garden, at least you know what it is - although you won't know how much active ingredient you're getting. Otherwise it's a total crapshoot, unless you have a gas chromatograph in your home lab.