Cordyceps sinensis (renamed now to Ophiocordyceps sinensis and also known as Caterpillar Mushroom, Cs-4, Dong Chong Xia Cao,…) has been used for hundreds of years as medicinal mushroom and is one of the most valued Chinese medicinal herbs . Cordyceps is used by some people as a stimulant and an “adaptogen”, which is used to reduce fatigue, enhance stamina and increase energy .
There are two human studies that yielded impressive results but before you get to excited it should be noted that they were only published in abstract form so it is difficult to know how well controlled they really were. Xiao and co-workers  found that six weeks of CordyMax (standardized extract of Cordyceps sinensis) supplementation significantly increased aerobic capacity in elderly. The second similar study by Zhu and Rippe  found that after 12 weeks of supplementation there were significant improvements in VO2max, work output and time to complete 1-mile walk. None of these studies were able to found the mechanism by which the substance improves performance.
On the other hand Allen C. Parcell and assistants found no improvements in oxidative capacity and endurance performance after 5 weeks of CordyMax supplementation in endurance-trained cyclists . There is also animal study by Balon et al.  which did not find performance improvements in swim times of rats fed by CordyMax (2 g/day) for 30 days. Although Balon et al. found no ergogenic effects they observe an improvement in insulin sensitivity in the treatment group after 10 days of supplementation. Sheree N. Colson and others  examined the effects of Cordyceps sinensis and Rhodiola rosea in male cyclists. The treatment group was given the herbal supplement Cordyceps sinensis and Rhodiola rosea, taking a loading dose of 6 capsules (2,000 mg) with water for 6 days followed by a maintenance dosage of 3 capsules for 7 days. There were no significant (p ≤ 0.05) differences in oxygen saturation, V̇O2max, ventilatory threshold, or time to exhaustion between treatment group and placebo.
At this time, it is premature to conclude that Cordyceps sinensis does or does not possess ergogenic properties due to many equivocal studies.
At this time there is not enough scientific information to determine an appropriate range of doses for cordyceps.
Zhu, Jia-Shi, Georges M. Halpern, and Kenneth Jones. “The scientific rediscovery of an ancient Chinese herbal medicine: Cordyceps sinensis Part I.” The Journal of alternative and complementary medicine 4.3 (1998): 289-303.
- Cordyceps – WebMD – Retrieved 18. March 2013
Xiao, Y., et al. “Increased aerobic capacity in healthy elderly humans given a fermentation product of Cordyceps Cs-4.” Medicine & Science in Sports & Exercise 31.5 (1999): S174.
Zhu, Jia-Shi, and James M. Rippe. “CordyMax enhances aerobic capability, endurance performance, and exercise metabolism in healthy, mid-age to elderly sedentary humans.” Gerontology 20 (2001): 297-298.
Parcell, Allen C., et al. “Cordyceps Sinensis (CordyMax Cs-4) supplementation does not improve endurance exercise performance.” International journal of sport nutrition and exercise metabolism 14.2 (2004): 236-242.
Balon, Thomas W., Arnie P. Jasman, and Jia-Shi Zhu. “A fermentation product of Cordyceps sinensis increases whole-body insulin sensitivity in rats.” The Journal of Alternative & Complementary Medicine 8.3 (2002): 315-323.
Colson, Sheree N., et al. “Cordyceps sinensis-and Rhodiola rosea-based supplementation in male cyclists and its effect on muscle tissue oxygen saturation.” The Journal of Strength & Conditioning Research 19.2 (2005): 358-363.
Walker, Thomas B. “Does Cordyceps sinensis Ingestion Aid Athletic Performance?.” Strength & Conditioning Journal 28.2 (2006): 21-23.