After reviewing existing clinical research on ginseng, British researchers concluded, "…there is compelling evidence for none of the claimed indications" (Vogler et al., 1999). The authors made this claim despite the fact that nine of the 16 papers reviewed reported a significantly positive effect. They reportedly weighed these positive results against contradictory findings in other studies, and also cited small study populations and the shortage of rigorously controlled and randomized trials for ginseng as reasons for their conclusion. On the other hand, it might be argued that the studies combined in this paper are too diverse to allow for meaningful conclusions.
Scientific literature for the review was culled from randomized, controlled clinical trials utilizing any species of ginseng for any indication. Species studied included Korean [Panax ginseng C.A. Meyer, Araliaceae]; Japanese [P. japonicus C.A. Meyer]; American [P. quinquefolius L.]; sanchi [P. notoginseng (Burk.) F.H. Chen.]; Vietnamese [P. vietnamensis Ha et Grushv.]; and eleuthero, a.k.a. Siberian ginseng [Eleutherococcus senticosus (Rupr. ex Maxim.) Maxim, Araliaceae]. Published and unpublished research in any language was considered. Only double-blind trials using single-ingredient root extract preparations were included. Sixteen studies utilizing P. ginseng, P. quinquefolius, or E. senticosus met the specified criteria and were reviewed.
The reviewers chose to lump different species of Panax and even Eleutherococcus together in their review, as if the plants' popular nomenclature carried more weight than their individual characteristics. They were clearly aware of the dissimilarities between Panax and Eleutherococcus, stating, "…their respective active constituents are different." The reviewed studies utilized a wide range of different standardized and unstandardized extracts of different ginseng species at various dosages.
Although the quality of each study was quantified using the Jadad scale (which evaluates trial methodology such as randomization, double-blinding, and other criteria), all seem to have been given comparable weight in the analysis, regardless of design, methodology, population size, extract used, treatment dose, or duration of treatment. A positive result for ginseng was reported in five of the six studies to which the authors assigned a Jadad score of 4. (Five is the highest possible Jadad score, which none of these studies achieved.) Studies that reported no significant effect were also the ones with the smallest ginseng treatment groups (generally eight to 10 participants). In every one of the larger studies (24 or more participants receiving any dose of ginseng), a positive effect was reported.
Seven of the 16 trials reviewed evaluated the effect of 200 to 400 mg/day P. ginseng extract or 618 to 1235 mg/day P. quinquefolius extract or 3.4 ml/day E. senticosus extract on physical performance in young, healthy volunteers. The studies were one to nine weeks in duration, and study groups consisted of nine to 14 participants per treatment. Three trials noted a significant ginseng effect during exercise (e.g., increased oxygen absorption, decreased heart rate, decreased levels of lactate in the blood) or after completing an exercise program (e.g., decreased body fat and resting heart rate, and increased muscle strength and maximal oxygen uptake). The four trials that reported no effect assessed similar, but not identical, outcome measures. Four of the seven trials also investigated ginseng's ability to increase tolerance to exercise-induced stress ("perceived exertion"), with no significant effect reported.
Of four studies that assessed cognitive function in young, healthy volunteers and one in elderly volunteers, three concluded that P. ginseng or E. senticosus significantly improved performance on tests of mental arithmetic, abstract thinking, inverted counting, association, or selective memory. The authors state, "Ginseng may have beneficial effects on psychomotor performance and cognitive behaviour." Two studies that investigated ginseng's ability to enhance immune function in healthy volunteers yielded contradictory findings - but again, different outcome measures were used. In one placebo-controlled study, there was a significantly greater increase in the number and activity of leukocytes (white blood cells) in people who took ginseng. A second, smaller trial detected no difference between treatment groups.
Positive effects were also reported in the final two of the 16 trials reviewed. In volunteers with type II diabetes mellitus, treatment with 200 mg/day ginseng (presumably P. ginseng) for eight weeks significantly improved participants' mood, vigor, psychophysical activity, and fasting blood glucose levels. A study of 93 people with herpes showed that after six months' treatment with an E. senticosus preparation, 75 percent of volunteers in the ginseng group reported improvement in the frequency, severity, and duration of their symptoms, compared with 34 percent in the placebo group. The reviewers devote a number of paragraphs to market data on ginseng, expressing concern that marketers make such a wide variety of claims for the herb. They conclude, "The widespread use of ginseng as a herbal remedy warrants more rigorous investigations to assess its efficacy and safety."