Are "designer" vitamins more potent than "generic" vitamins? I had a dietitian tell me that I should never buy vitamins from places like Walmart, Walgreens, or similar stores because the potency is never guaranteed. She said that the potency greatly varies from pill to pill and sometimes is only 25% as potent as listed on the label. She directed me to online stores that sell awfully expensive vitamins that "guarantee" the potency. I checked the nutritional labels between both the Walmart and "designer" brands and they both have the exact same dosage per pill, the only difference is brand and price. Her claim is that I'm simply wasting money on these generic vitamins and I am simply taking a pill that mainly contains inert ingredients. I worked in the breakfast foods industry for a few years and I know that the FDA would be all over us if we lied on our product labels. Is there any evidence that the generic, cheap vitamins are less potent than more expensive vitamin preparations?

The concept is not entirely a sham. The formal name is "Bioavailability". It doesn't matter how much of a vitamin is present in a pill, when it isn't used by the body. Yet the label lists what's present in the pill.

However, evolution has made us quite efficient in getting vitamins from food, where they're relatively unavailable because they're locked up inside cells. It would take malice to make pure vitamin C not bio-available. It's a very simple molecule. Vitamin B12 is notorious, on the other hand. It must be actively extracted by your intestines. In that case, pills are still not a good idea: B12 availability is capped per meal. A high peak (i.e. from a pill) wouldn't be absorbed (low bio-availability) because your body doesn't have the capacity to deal with the excess.