St. John's wort has been shown to be effective in treating mild to moderate forms of major depression, but problems with supplement quality have been a concern. Among the 10 St. John's wort supplements selected for testing by ConsumerLab.com in 2010, for example, six failed to meet ConsumerLab.com's quality standards due to contamination and/or a lack of expected ingredients. However, in ConsumerLab.com's latest review, nine of 10 currently marketed St. John's wort supplements passed testing. Americans purchased $54 million of St. John's wort supplements in 2011 according to Nutrition Business Journal, down from a high of $315 million in 1998.

The St. John's wort plant is known to accumulate heavy metals from the soil, such as cadmium and lead. All products were tested for these metals as well as for levels of the St. John's wort marker compounds hypericin or hyperforin. All products passed these tests. ConsumerLab.com also tested tablets and caplets for their ability to properly disintegrate within the 30 minute USP requirement. One of the supplements failed this test, taking 55 minutes—suggesting that the pills were not properly made and the ingredients may not be fully available for absorption.

ConsumerLab.com's Product Review of St. John's Wort Supplements is available at consumerlab.com/reviews/St_Johns_Wort/stjohnswort and includes results for the ten supplements selected by ConsumerLab.com as well as for eight others which passed the same evaluation through ConsumerLab.com's Quality Certification Program. Also listed are two products similar to ones that passed but which are sold under different brand names. To help consumers find the best St. John's wort supplement, the report provides quality ratings, ingredient comparisons, and cost comparisons. The report includes tips on buying and using these supplements and important information about potential side effects and drug interactions with St. John's wort.

Brands included in the report are Finest Nutrition (Walgreens) St. John's Wort, Gaia Herbs St. John's Wort, Flower Buds & Tops, GNC Herbal Plus Standardized St. John's Wort, Kira St. John's Wort, Metagenics St. John's Wort with Folate and B12, Natural Factors St. John's Wort, Nature's Answer Super St. John's Wort, Nature's Bounty St. John's Wort, Nature's Herbs Power-Herbs St. John's-Power, Nature's Way Perika St. John's Wort, New Chapter St. John's Force, Paradise St. John's Wort, Planetary Herbals Full Spectrum St. John's Wort, Puritan's Pride St. John's Wort 300 mg, Solaray St. John's Wort, Solgar St. John's Wort '300', Spring Valley (Walmart) St. John's Wort 0.3%, Vitamin Shoppe St. John's Wort Extract, Vitamin World St. John's Wort, and Vitanica St. John's Wort.