When you reach for that bottle of vitamin C or fish oil pills, you might wonder how well they’ll work and if they’re safe. The first thing to ask yourself is whether you need them in the first place.

More than half of all Americans take one or more dietary supplements daily or on occasion. Supplements are available without a prescription and usually come in pill, powder or liquid form. Common supplements include vitamins, minerals and herbal products, also known as botanicals.

People take these supplements to make sure they get enough essential nutrients and to maintain or improve their health. But not everyone needs to take supplements.

Some supplements may have side effects, especially if taken before surgery or with other medicines. Supplements can also cause problems if you have certain health conditions. And the effects of many supplements haven’t been tested in children, pregnant women and other groups. Be sure to talk with your health care provider if you’re thinking about taking dietary supplements.

Evidence does suggest that some supplements can enhance health in different ways. The most popular nutrient supplements are multivitamins, calcium and vitamins B, C and D. Calcium supports bone health, and vitamin D helps the body absorb calcium. Vitamins C and E are antioxidants — molecules that prevent cell damage and help to maintain health.

Women need iron during pregnancy, and breastfed infants need vitamin D. Folic acid — 400 micrograms daily, whether from supplements or fortified food — is important for all women of childbearing age.

Vitamin B12 keeps nerve and blood cells healthy. Research suggests that fish oil can promote heart health. The health effects of some other common supplements need more study. These include glucosamine (for joint pain) and herbal supplements such as echinacea (immune health) and flaxseed oil (digestion).

Many supplements have mild effects with few risks. But use caution. Vitamin K, for example, will reduce the ability of blood thinners to work. Ginkgo can increase blood thinning. The herb St. John’s wort is sometimes used to ease depression, anxiety or nerve pain, but it can also speed the breakdown of many drugs — such as antidepressants and birth control pills — and make them less effective.

Just because a supplement is promoted as “natural” doesn’t necessarily mean it’s safe. The herbs comfrey and kava, for example, can seriously damage the liver.

For vitamins and minerals, check the % Daily Value (DV) for each nutrient to make sure you’re not getting too much.

Because supplements are regulated as foods, not as drugs, the FDA doesn’t evaluate the quality of supplements or assess their effects on the body. If a product is found to be unsafe after it reaches the market, the FDA can restrict or ban its use.