Dietary supplements that contain Ma Huang (ephedra alkaloids) and guarana (caffeine) are widely marketed and used in the U.S. for weight loss and athletic performance enhancement, despite a lack of adequate research on the pharmacology of these botanical stimulants. We developed and applied a novel liquid chromatography-tandem mass spectrometry (LC-MS-MS) method to quantitate the various ephedra alkaloids found in dietary supplements that contain Ephedra species. The quantities of ephedrine, pseudoephedrine, norephedrine, norpseudoephedrine, methylephedine, methylpseudoephedrine, and caffeine were determined for 35 commercial dietary supplements and compared with the amounts listed on the product labels. The total ephedra alkaloid content ranged from 5.97 mg to 29.3 mg per serving. Two supplement brands did not list the quantity of ephedra alkaloids on the label, and four did not list the amount of caffeine per serving. Of the products tested, 31% contained > 110% of the total ephedra alkaloids listed on the label, and 6% of the supplements contained < 90% of the listed amount. For caffeine, 86% of the product lots that listed the caffeine amount contained less than 90% of the labeled quantity. No products contained > 110% of the declared caffeine content. The total ephedra alkaloid content varied significantly from lot to lot in 5 of 9 products. Three product brands contained proportions of alkaloids that exceeded amounts reported for E. sinica, including one that was 98% ephedrine, one that had 10% norpseudoephedrine, and one that contained an average of 13% methylephedrine. We conclude that product inconsistency is common among some commercially available dietary supplements that contain ephedra alkaloids and caffeine.