You buy vitamins and other nutritional supplements with the goal of improving your health, but do you know exactly what to look for, or what's inside the bottle? Just because a supplement is labeled "all-natural" doesn't mean it's safe -- or effective.

Before you buy any supplement, read through this list of frequently asked questions to make sure you're buying a product that helps rather than harms your health.

What Is a Dietary Supplement?

Dietary supplements include vitamins, minerals, herbs, botanicals, enzymes, amino acids, or other dietary ingredients. You take these products by mouth in pill, capsule, tablet, or liquid form to supplement your diet.

Can I Take Supplements on My Own, Without a Doctor?

Supplements are available for sale over the counter at your local pharmacy or online without a prescription. Still, you should always check with your doctor before taking any product, because some supplements can cause side effects, or interact with other prescribed or over-the-counter medicines or supplements you're already taking. It's especially important to ask your doctor about taking a supplement if you're pregnant or nursing, about to have surgery, or you have a health condition such as high blood pressure, heart disease, or diabetes. Also, don't give a supplement to a child without checking with their health care provider.

How Do I Take Supplements?

Use common sense when taking any dietary supplement. These simple tips can help you stay on track:

Follow the directions on the package and your doctor's instructions exactly.

Keep a list of all supplements and other medications you're taking.

Write down how much of each product you're taking and at what time of day. It’s easy to forget that you’ve taken a supplement, which can lead to accidentally taking too much.

Also write down how the supplement affects you, and whether you have any side effects.

Share your notes with your doctor at each visit.

Are All Supplements Tested to Make Sure They're Safe and Effective?

No. Manufacturers aren't required to test their products for safety and effectiveness. Some supplement ingredients have been tested in animal or human studies. For example, folic acid has been shown in studies to reduce the risk of birth defects in pregnant women. However, other supplement ingredients haven't been studied well, or at all.

The U.S. Food and Drug Administration (FDA) does regulate dietary supplements; however, it treats them like foods rather than medications. Unlike drug manufacturers, the makers of supplements don’t have to show their products are safe or effective before selling them on the market.

How Do I Know Whether a Supplement's Claims Are True or False?

Supplement makers are not allowed to claim their product diagnoses, treats, cures, reduces the symptoms of, or prevents disease -- and there needs to be a disclaimer statement to that effect on the label. Look for overblown claims on the label or box, such as "totally natural," "completely safe," or "miracle cure." If you're unsure about a product, ask your doctor or pharmacist. Or, call the supplement manufacturer and ask them what studies they've done to support the claims they're making.

What Is a ‘Proprietary Blend’?

A "proprietary blend" is a combination of ingredients used exclusively by one supplement manufacturer. No other company produces the exact same combination of ingredients, and, in most cases, it is difficult to know from the label the exact amounts of each of the ingredients in that blend.

What Is the Difference Between RDA and DV?

Recommended Dietary Allowance (RDA) is the amount of a certain nutrient you should get each day based on your age, gender, and whether you are pregnant or breastfeeding. On a supplement label, you're more likely to see the acronym DV, which stands for Daily Value. This represents how much of a nutrient the supplement provides in regards to a total daily diet. For example, if a calcium supplement is labeled "50% DV," it contains 500 mg of calcium per serving, because the DV for calcium is 1,000 mg per day. Sometimes the DV contained in a supplement will be higher than the RDA for certain people. In many cases, there is no DV for a supplement, so the label will reflect that. Check with your doctor to make sure your supplement doesn't contain too much of any nutrient.

What Should I Do if I Have a Side Effect From a Supplement?

Report any side effects to your doctor, and to the FDA, as soon as possible. You can reach the FDA at 800-FDA-1088, or go to www.fda.gov/med watch to report a problem.