You stroll down the pharmacy aisles on what seems like a simple mission: pick up some vitamins. But a quick glance at a bottle's label can send you running for a dictionary. Things like "RDA" or "DV" are just a few examples of an alphabet soup that's on many packages. But don't despair. We'll help you demystify supplement guidelines.

What the Numbers Mean

Many of the terms you see on labels or supplement web sites can help you understand how much of the vitamin or mineral you should take. For example, here are some guidelines set up by the Institute of Medicine:

The RDA (Recommended Dietary Allowance) and the AI (Adequate Intake) are the amounts of a vitamin or mineral you need to keep healthy and stay well-nourished. They're tailored to women, men, and specific age groups.

The UL (Tolerable Upper Intake Level) is the maximum amount of daily vitamins and minerals that you can safely take without risk of an overdose or serious side effects. For certain nutrients, the higher you go above the UL, the greater the chance you'll have problems.

Separate from the RDA and the UL, the Food and Drug Administration uses a different measure for the nutrients you need:

The DV (Daily Value) is the only measurement you'll find on food and supplement labels. That's because space is limited, and there's a need for one single reference number. That number is the amount of a vitamin or nutrient that you should get for top health from a diet of 2,000 calories a day. The DV is sometimes the same as the RDA.

Although the details may be different, remember that the RDA and DV are both set up to help you get the nutrients you need to prevent disease and avoid problems caused by lack of nutrition.

How Much Is Too Much?

Because high doses of some supplements can have risks, how do you know when it's OK to take more than the RDA or DV?

One way is to look for the UL (tolerable upper intake level) of a nutrient. With many vitamins and minerals, you can safely take a dose much higher than the RDA or DV without coming close to the UL.

For instance, the average person can take more than 50 times the RDA of vitamin B6 without reaching the upper limit. But some people develop symptoms of nerve pain with these higher levels of B6. So you should always be cautious. Here are some things to keep in mind:

Some supplements are riskier than others. With some vitamins and minerals, the upper limit is pretty close to the RDA. So it's easy to get too much. For example, a man who takes just over three times the RDA of vitamin A would get more than the upper limit. High doses of vitamin A -- and other fat-soluble vitamins like E and K -- can build up in the body and become toxic. Other risky supplements include the minerals iron and selenium.

Supplements are designed to be additions to your diet. Popping pills is not the answer to good health. Experts say you should eat a well-balanced diet and take supplements to fill in any nutritional gaps. Some people take a multivitamin with minerals for nutritional insurance.

The UL is often the limit for all sources of a nutrient. It can include the amount you get from both food and supplements. So when you figure out whether you've reached the UL on a particular nutrient, take into account the food you eat.

You won't find the UL on food nutrition labels or on your vitamin bottle. It's not a number that most people know about. But you'll see it on government web sites. And there's a complete list of nutrients with ULs at the end of this article.

Most supplements don't have a UL -- or RDA or DV. The government has only set levels for a fraction of the vitamins and supplements available. For most of the supplements you see on the shelves, experts really don't know the ideal or maximum dose.

Many nutrients, in too high a dose, can be dangerous. To be on the safe side, steer clear of the UL for any nutrient. And if you have a health condition, check with your doctor before you take supplements. He can tell you if they have side effects or interfere with other medicines you use.

Table: RDAs and ULs for Vitamins and Minerals

The Institute of Medicine has determined upper limits for 24 nutrients. This table is for adults ages 19 or older. It doesn't apply to women who are pregnant or breastfeeding, because they have different nutritional requirements.