A study carried out by the independant company ConsumerLab.com found that many multivitamins actually contain significantly higher or lower levels of vitamins and minerals than they claim on the packet.

ConsumerLab reported that they found defects in 30% of the multivitamins that they selected for testing. They considered the actual contents of the pills against the Institute of Medicine's standards and highlighted some of the results.

Three of four popular children's multivitamins were too high in vitamin A. One men's multivitamin was contaminated with lead and another had too much folic acid -- associated with more than doubling the risk of prostate cancer.

One general multivitamin had only 50% of its folic acid. Another was missing 30% of its calcium.
A senior's, a prenatal, and a women's multivitamin each had only 44.1%, 44.3%, and 66.1%, respectively, of their vitamin A.

A vitamin water had 15 times its stated amount of folic acid, so drinking one bottle would exceed the tolerable limit for adults; less than half a bottle would put children over the limit.  A pet multivitamin was contaminated with lead and another had only 46% of its vitamin A and 54.7% of its calcium.

The full report is only available if you're a ConsumerLab.com subscriber, but these statistics alone would give me pause for thought. If you do take a multivitamin, talk to your doctor or pharmacist to make sure that it's a reputable brand (going for the cheapest option could mean that you risk taking a pill that doesn't contain what it says on the packet).

The safety of dietary supplements isn't a new topic: two years ago, Diet Blog published Diet Supplements: Asking the Hard Questions. And as Mike writes in Which Supplements Actually Work?: I believe certain supplements can be beneficial, but before considering them, look at your diet first.

Don't take vitamin supplements as an excuse not to bother eating at least five daily portions of fruits and veggies (and aim for seven or eight if you can), and do discuss any pills you're taking with your doctor - especially if you're elderly or pregnant.