Why Herb Studies Disagree: Extracts, Plant Chemistry and Product Variability
Why studies of the same herb can disagree, from plant species, parts and growing conditions to extraction and the amounts of marker compounds found in products.
Content last updated . This page has not been medically reviewed.
Safety first. Supplements can cause side effects and interact with medicines. Read mushroom safety and interactions before you use any mushroom product.
Studies of the "same" herb often disagree because they are rarely testing the same thing. Plant species, the part used, growing and storage conditions, how the extract is made and what ends up in a finished product can all differ. A result for one product does not automatically carry over to another. This page lists the documented reasons, with the studies behind them. It does not say which products to buy.
Documented reasons results differ
| Reason | What the source found | Study type |
|---|---|---|
| Different species and plant parts | A Cochrane review found that echinacea trials used preparations based on different species and parts of the plant. [1] It declined to pool them because of strong clinical heterogeneity. [1] | Systematic review of 24 trials |
| Growing conditions | NCCIH reports that recent research it sponsored suggests that the activity of echinacea extracts is influenced by soil conditions that affect the plant's bacterial community. [2] | Laboratory and field research |
| Environment, drying and storage | Analysis of Italian St. John's wort (Hypericum perforatum L.) samples found hyperforin ranging from 1.37% to 20.80%. The authors evaluated the effect of environmental factors and drying and storage conditions on the wild samples. [3] | Laboratory analysis |
| Extract content between brands | An analysis of ginkgo (Ginkgo biloba L.) products on the U.S. market found flavone glycosides ranging from 24% to 36% and terpene lactones from 4% to 11%. [4] | Laboratory analysis |
| Label versus bottle | A study of 25 ginseng products found marker compounds differed significantly from labeled amounts, and ginsenoside concentrations varied by 15- and 36-fold in capsules and liquids. [5] | Laboratory analysis |
| Chemical forms within a species | A review of mint (Mentha) essential oils analyzes the structural variability of the oils across major Mentha species and links it to chemotypes. [6] | Review |
| How the extract is made | In the saw palmetto review, hexane-extracted products and other products showed no difference in effect on symptoms. [7] | Systematic review |
| Study product versus shelf product | Supplements you buy may differ in important ways from products tested in studies. [8] | Agency guidance |
| Incomplete reporting | Key herbal details, such as the percentage of active constituents and the form of the preparation, may be missing from trial reports. [9] | Review of reports |
Several of these laboratory analyses are 20 or more years old (the ginseng, ginkgo and St. John's wort papers were published in 2001 and 2002). The sources we read do not describe how much today's U.S. products vary, so these figures show that variation has been measured, not what any current product contains.
How the terms are used
- Marker compound. In the ginseng study, ginsenosides were the marker compounds measured for Panax species. [5]
- Standardized. The authors of the ginseng study suggest standardization may be necessary for quality assurance. [5]
- Chemotype. The mint review uses this word for chemical diversity within a plant group. [6] We did not find a plain definition in the sources we read, so we do not offer one here.
- Heterogeneity. In the echinacea review, strong clinical heterogeneity (large differences between studies) was the reason the authors did not pool the trials. [1]
Our pages on standardized extracts and marker compounds cover the product side in more depth.
What this means for evidence
- A positive or negative trial applies to the product tested. NCCIH says an herbal supplement may contain dozens of chemical compounds and not all of its ingredients may be known. [10]
- Proper evidence not found for a rule that says "extract A is better than extract B" across herbs. We searched the NCCIH pages and the PubMed studies cited here. Each compares products only for a single herb and measurement.
- Disagreement between trials is not proof that an herb is useless or that it works. It is a reason to ask what exactly was tested.
Frequently asked questions
Why did one echinacea trial work and another not?
The trials used different species, parts and preparations. [1] NCCIH-sponsored research also suggests growing conditions can affect extract activity. [2]
Are ginkgo brands the same?
Not in the 2002 U.S. market analysis, which found flavone glycosides from 24% to 36% and terpene lactones from 4% to 11%. [4] Current products may differ.
Does a standardized label guarantee a match with the studies?
Proper evidence not found. The sources we read show label-to-bottle gaps in older analyses [5] and say supplements may differ from products studied. [8]
Does this mean all supplements are unreliable?
No. The studies we read measured specific herbs and products. They do not describe all supplements.
What should I do with this?
Use it as a reason to ask a pharmacist about your own product and medicines. See the questions to ask your pharmacist and how herb trials are designed.
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
Safety and quality
A product that differs from its label may contain more or less of a compound than expected. That matters for people who take medicines, are pregnant or breastfeeding, or have liver or kidney conditions, and for people facing surgery. Ask a pharmacist or clinician first. Read about adulteration and mislabeling, quality testing and certifications, and third-party testing for herbal products.
References
- Karsch-Völk M, Barrett B, Kiefer D, Bauer R, Ardjomand-Woelkart K, Linde K. Echinacea for preventing and treating the common cold. Cochrane Database Syst Rev. 2014. PMID 24554461. Read 2026-10-22.
- National Center for Complementary and Integrative Health (NCCIH). Echinacea. Last updated November 2024. Read 2026-10-22.
- Bergonzi MC, Bilia AR, Gallori S, Guerrini D, Vincieri FF. Variability in the content of the constituents of Hypericum perforatum L. and some commercial extracts. Drug Dev Ind Pharm. 2001. PMID 11548855. Read 2026-10-22.
- Kressmann S, Müller WE, Blume HH. Pharmaceutical quality of different Ginkgo biloba brands. J Pharm Pharmacol. 2002. PMID 12005361. Read 2026-10-22.
- Harkey MR, Henderson GL, Gershwin ME, Stern JS, Hackman RM. Variability in commercial ginseng products: an analysis of 25 preparations. Am J Clin Nutr. 2001. PMID 11382666. Read 2026-10-22.
- Yu Y, Ma Y, Ouyang Z, Huang L. Mentha essential oils: unraveling chemotype-dependent biosynthesis and assessing evidence for health-promoting activities. Nutrients. 2025;17(20):3258. PMID 41156510. Read 2026-10-22.
- National Center for Complementary and Integrative Health (NCCIH). Saw Palmetto. Last updated April 2025. Read 2026-10-22.
- National Center for Complementary and Integrative Health (NCCIH). Using Dietary Supplements Wisely. Read 2026-10-22.
- Gagnier JJ, DeMelo J, Boon H, Rochon P, Bombardier C. Quality of reporting of randomized controlled trials of herbal medicine interventions. Am J Med. 2006. PMID 16945616. Read 2026-10-22.
- National Center for Complementary and Integrative Health (NCCIH). Natural Doesn't Necessarily Mean Safer, or Better. Read 2026-10-22.