Don’t count on dietary supplements to contain the dose listed on the label. Over the years, third-party analyses have found substantial variability in most types of supplements. Vitamin D was the focus of attention of a report in JAMA Internal Medicine in 2013.
Researchers tested 12 brands of vitamin D and found they contained anywhere from 52 to 135 percent of the labeled dose, sometimes with wide variability in pills from the same bottle. One-third of the brands were off by more than 10 percent, which is considered the acceptable cutoff.
To improve the odds of getting what you pay for, look for brands with the “USP Verified” seal from the nonprofit U.S. Pharmacopeial Convention, though not many products apply for it. In this study, the one brand with the seal was most accurately labeled.
Keep in mind, lack of accuracy in dosage usually doesn’t matter, since for most supplements the “right” dose isn’t known and the proposed benefits are unproven.