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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?

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    The FDA does not regulate dietary supplements, it only requires that all ingredients be mentioned on the label – JoseK Jul 11 '11 at 7:00
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    Doesn’t answer the question, but: do you need vitamin supplements at all? Apparently their effectiveness has been greatly exaggerated in the past and with very few exceptions you get all the vitamins you need from your normal food, no supplements needed (and in fact, taking more vitamins doesn’t help, the body simply ejects them). – Konrad Rudolph Jul 11 '11 at 9:24
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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.

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As was mentioned in a comment, the FDA does not provide strict regulation of dietary supplements. The FDA allows questionable fillers and binders such as; cork by products, chemical FD&C dyes, sodium benzoate, dextrose, ethycellulose, and propylene glycol to name only a few. These additives, in addition to the quality of the actual desired supplement, may decrease your ability to absorb the vitamin.

This impacts the concept of Bioavailability mentioned in @MSalter's answer.

Additionally, there are distinct grades of vitamins.

Pharmaceutical Grade Vitamins meet the standards set by the US Pharmocepia guidelines (USP) for dissolution of the capsule or tablet.

Vitamins that are not identified as Pharmeceutical Grade are typically Food Grade, which merely means that it is safe for human consumption.

The upshot is, you really won't be able to guarantee the purity, strength, quality or identity of the ingredients within an over the counter vitamin supplement whereas pharmaceutical grade vitamins need to contain exactly what they say they contain in the measurements stated.

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