Here is a look at some of the most popular supplements and what scientific studies show regarding their safety and effectiveness:

Fish oil, omega-3 fatty acids

Source: Fatty fish, such as bluefish, halibut, mackerel provide 2 of 3 fatty acids; the third comes from walnuts, olive oil and flaxseed oil.

Health claim: Mainly prevention of heart disease and related problems.

Does it work? Several studies offer evidence fish oil reduces heart disease. Evidence is mostly lacking for treatment or prevention of numerous other conditions, such as cancer, mental decline and dementia and arthritis relief.

Some health organizations encourage omega-3 consumption from specific foods or supplements.

Children should not take fish oil capsules.

Side effects: Minimal to none. However, high doses can cause bleeding, especially if used with aspirin and warfarin.

Source material: Agency for Healthcare Research & Quality and NIH Medline Plus

Glucosamine and Chondroitin sulfate

Source: Glucosamine is usually obtained from the shells of shrimp, lobster, and crabs. Chondroitin sulfate is from the cartilage of sharks and cattle. Synthetic versions can also be made in the laboratory.

They are sold individually and often in combination with each other.

Health claim: Arthritis pain, usually in the knees.

Does it work? Not for most people with mild pain who took part in the government's largest study of these supplements; dummy pills were just as effective.

The supplements did help some people with moderate to severe pain, but researchers said the group was too small to be convincing.

Another caveat: No supplements on the market were of high enough quality to be used in the study. Researchers ordered up their own batch that meet purity and potency standards.

Side effects: Minor, sometimes upset stomach during the six-month study.

Source material: National Center for Complementary and Alternative Medicine

Echinacea

Source: Coneflower.

Health claim: Mostly to treat or prevent colds.

Does it work? Two government studies found it didn't work for treating colds in children or adults. Some other studies suggested possible benefit, and the head of the U.S. alternative medicine institute wants more study.

Most studies show echinacea doesn't prevent colds.

Side effects:

Usually none, but there can be stomach problems, rashes, increased asthma. People with ragweed and similar allergies can have allergic reactions.

Flaxseed oil

Source: Flaxseed, also known as linseed.

Health claim: Usually as a laxative. Also for hot flashes and breast pain, high cholesterol, cancer prevention.

Does it work? Flaxseed contains soluble fiber and is an effective laxative. Its effectiveness at preventing heart disease and cancer are unproven. Study results are mixed on whether flaxseed might reduce hot flashes.

Side effects: Flaxseed should be taken with plenty of water or it may worsen constipation or, in rare cases, even cause intestinal blockage. Flaxseed may lower your ability to absorb oral medications. Flaxseed should not be taken at the same time as other medications or supplements.

Source material: National Center for Complementary and Alternative Medicine.

Ginseng

Source: Root of Asian or American ginseng, an herb native to Korea and China. It is used in tablets, capsules, extracts, teas, as well as creams. (Siberian ginseng or eleuthero is not a true ginseng.)

Health claim: Immune system booster, improvement of stamina, physical and mental performance; lowering blood sugar and blood pressure.

Does it work? There is no convincing evidence ginseng works for anything. One study suggested it helped cancer patients relieve fatigue, but doctors said it was too soon to recommend it; also, the ginseng used in the study was powdered American ginseng root whose purity was verified by the scientists. Some studies suggest ginseng may lower blood sugar.

Side effects: Usually none. Most common are headaches and sleep and gastrointestinal problems; some reports of breast tenderness, menstrual irregularities, and high blood pressure, but these products were not analyzed, so effects may have been due to another herb or drug in the product.

Ginseng may lower levels of blood sugar, so diabetics should be extra cautious, especially for interactions with medications or other herbs.

Source material: National Center for Complementary and Alternative Medicine, American Society of Clinical Oncology.

Ginkgo, or ginkgo biloba

Source: Leaves of the ginkgo tree are used to make tablets, capsules, or teas.

Health claim: Memory improvement, dementia or Alzheimer's disease treatment

Does it work? A large study of the elderly over six years found it ineffective at preventing dementia and Alzheimer's. A short-term study showed it didn't improve memory in people over 60.

Side effects: It may increase bleeding risk, so people who use blood thinners or who have bleeding disorders, or have scheduled surgery or dental procedures should use caution and discuss with a doctor. Headache, nausea, stomach upset, diarrhea, dizziness or allergic reactions have been reported.

Also, uncooked ginkgo seeds - not the leaf extract - contain a chemical that can cause seizures. Consuming large quantities of seeds over time can be deadly.

Source material: National Center for Complementary and Alternative Medicine

Garlic

Source: Garlic cloves. They can be eaten raw or cooked, used to make extracts or dried to produce a powder or capsules.

Health claim: Lowering cholesterol, preventing heart disease, high blood pressure. Prevention of stomach and colon cancers.

Does it work? A government-funded study found it had no effect at lowering cholesterol. Another study on the long-term use of garlic supplements to prevent stomach cancer found no effect. Some preliminary research suggests garlic may slow hardening of the arteries and lower the risk of some cancers, but this has not been rigorously tested.

Side effects: Garlic appears safe for most adults. However, it can cause breath and body odor and upset stomach. It can thin the blood so it should be avoided for at least a week before surgery. It can interfere with the effectiveness of saquinavir, a drug used to treat HIV infection. Its effect on other drugs has not been well studied.

Source material: National Center for Complementary and Alternative Medicine