Every year, supplements send roughly 20,000 people to the emergency room.
Last year, the world's largest dietary supplement maker, GNC Holdings Inc, agreed to pay $2.25 million to avoid federal prosecution over its alleged sale of illegal pills and powders.
And the Food and Drug Administration has ordered the makers of several supplements to recall their products after scientists found traces of illegal and potentially dangerous molecules in their formulas.
Still, while many supplements are useless, there are others that we can't enough of simply by eating a healthy diet.
So here are the supplements you should take — and the ones to avoid.
Protein powder: Get it — eat beans, tofu, nuts, fish, or meat instead.Marketed as necessary for weight gain and muscle building, protein is one of the best-selling supplements in the US.
Protein is good for you — it helps build muscles — but most Americans get plenty in their diets. In fact, most of us get too much. Meat, fish, beans, tofu, and nuts are rich in protein. Plus, numerous companies have been accused of spiking their protein powders with cheap fillers — another reason to avoid the powdered stuff.
Homeopathic remedies: Skip them — they don't work.
Advocates of homeopathy — which involves diluting an active ingredient until there's no measurable quantity left — claim that the treatments can do everything from relieving colds to calm anxious pets.
But homeopathy has repeatedly been shown to be ineffective. A 2005 study published in the medical journal The Lancet found the approach was roughly as effective as a placebo.
Workout boosters like Jack3d or OxyElite Pro: Skip them — they've been linked to illness and at least one death.
For years, the makers of these supplements, whose active ingredient is dimethylamylamine (DMAA), claimed that they increased speed, strength, and endurance.
But in 2011, after two soldiers who used Jack3d died, the US Department of Defense removed all products containing DMAA from stores on military bases. A 2015 indictment against Dallas company USPlabs, which makes OxyElite Pro, accused the company of falsely claiming that its product was made of natural plant extracts. In reality, it contained synthetic stimulants made in China. The indictment also claimed that the use of OxyElite led to several liver injuries and at least one death.
Zinc: Take it — it's one of the only ingredients shown to shorten colds.
Zinc seems to interfere with the replication of rhinoviruses, the bugs that cause the common cold.
In a 2011 review that looked at studies of people who'd recently gotten sick, researchers compared those who'd started taking zinc with those who just took a placebo. The ones on zinc had shorter colds and less severe symptoms.
Creatine: Get it — Unless you eat red meat instead.
We all produce natural, low-level amounts of creatine, a compound that helps our muscles release energy. Studies show that we produce more of it when we eat meat regularly.
Research suggests that taking creatine supplements could have moderate benefits on specific kinds of short-intensity workouts. It appears to help muscles make more adenosine triphosphate (ATP), a chemical-energy transporter. But unsurprisingly, there's no evidence that it's beneficial for other types of exercise involving endurance or aerobics.
Weight-loss pills like "Hydroxycut": Skip them — their claims are dubious.
Weight-loss supplements like Hydroxycut claim that they can help you slim down with a boost of "pro clinical" ingredients. The formula once contained Ephedra, a powerful stimulant linked to 155 deaths that was banned by the US Food and Drug Administration in 2003.
Today's ingredients are simply caffeine and four herbal extracts: Lady's mantle, wild olive, komijn, and wild mint. Several studies show caffeine can help boost metabolism and encourage moderate, short-term fat burning. But no long-term studies show caffeine helps with sustained weight loss.
Folic acid: Take it if you're pregnant or want to get pregnant.
Our bodies use folic acid to make new cells. The National Institutes of Health recommends that women who are currently pregnant or who want to get pregnant take 400 micrograms of folic acid daily since their bodies demand more of this nutrient when carrying a growing fetus.
Several large studies have also linked folic acid supplementation before and during pregnancy with decreased rates of neural-tube defects, which are serious and life-threatening defects of a baby's brain, spine, or spinal cord.
Green-coffee extract: Skip it — the only study backing it was pulled.
"You may think magic is make-believe, but this little bean has scientists saying they've found the magic weight-loss cure for every body type," Dr. Oz said of green-coffee extract on his show in 2012.
Unfortunately, there was only one study backing green coffee's alleged weight-loss capabilities, and it was funded by the extract's manufacturer. The study was retracted a few months later.
Green-tea extract: Try it — it's been linked with some health benefits, and is generally considered safe.
A series of preliminary Mayo Clinic studies conducted in 2010 showed promise for the potential use of a chemical component of green tea (epigallocatechin gallate) in reducing the number of cancer cells in patients with chronic lymphocytic leukemia. Research on green tea consumption in people with other forms of cancer has been too limited to say whether it's beneficial.
Brewing and drinking green tea is the easiest way to get the extract, but it's also added to foods like yogurt and other beverages, or available in pill form.
Gingko Biloba: Skip it — the studies don't prove it helps.
Ginkgo biloba, which comes from the maidenhair tree, is one of the best-selling products in the US for memory loss and is often marketed as a "brain booster."
But the evidence is inconsistent. A small 2006 study found ginkgo was as effective as the drug donepezil for boosting attention and memory in people with mild to moderate Alzheimer's. But a large 2008 study of healthy older people found no evidence that ginkgo helped to prevent dementia, including Alzheimer's. A 2009 follow-up study also found no evidence that gingko slowed cognitive decline or memory loss.