A study of Rhodiola supplements (including hard capsule extracts, softgel capsules, tablets, and liquid extracts) in Great Britain found that nearly 20% did not contain rosavin, the key marker compound used to identify Rhodiola rosea from other Rhodiola species, and many others appeared to be adulterated with other Rhodiola species.
The identities of the tested products were not disclosed, so it is not clear whether any are available for purchase in the U.S.
[UDPATE (3/18/16): ConsumerLab.com published it own review of rhodioloa supplements sold in the U.S. and found that 22% of the products it selected for testing failed to contain the expect amounts of rosavin compounds. Details, including product names and comparisons, and CL's "Top Picks," are available in the Rhodiola Rosea Supplements Review >>]
In Europe, herbal products must be registered under the Traditional Herbal Medicine Products Directive (THMPD), which requires certain manufacturing and quality standards. Two of the 40 Rhodiola products in the study were THMPD registered and were found to contain authentic Rhodiola rosea. However, unregistered products are still available on the European market, including the 38 unregistered Rhodiola products in the study.
The authors noted that due to the herb's popularity and a lack of large-scale cultivation, there may be an insufficient amount of authentic R. rosea available to meet current demands -- which, in some cases, can lead to adulteration with other Rhodiola species.
Rhodiola rosea supplements may be modestly helpful for depression and mild anxiety and are promoted for other uses, such as enhancing mental function and improving sports performance (see the Encyclopedia article about Rhodiola rosea for more information).
To read the study abstract, use the red link below.