Nearly half of us take herbal or dietary supplements daily, and these over-the-counter products are a booming industry. Herbal dietary supplement sales reached $6 billion yearly, based on the most recent estimates by the American Botanical Council.

Certain supplements may improve your health, but others can be ineffective or even harmful.

"Buyer beware,” warns JoAnn Manson, MD, chief of the division of preventive medicine at Brigham and Women’s Hospital in Boston, and professor of medicine at Harvard Medical School. “Many supplements on the market have not been rigorously tested. Very few supplements have shown to be of benefit," says Dr. Manson. She adds that many carry unsubstantiated health claims.

Here are four supplements you should take carefully, if at all:

1. Vitamin D: Too Much Can Harm Your Kidneys

Vitamin D promotes calcium absorption in the body, and getting enough is central to health and wellbeing. Supplemental vitamin D is popular, offering the promise of protecting bones and preventing bone diseases like osteoporosis. But in many cases, healthy post-menopausal women who take low-dose vitamin D supplements (up to 400 international units, IU) might not actually need them.

As Manson notes, enthusiasm for high-dose vitamin D supplements is outpacing evidence. “More is not necessarily better when it comes to micro-nutrient supplements,” says Manson.

After looking at the evidence, it turns out that when healthy women take low doses of vitamin D, it does not necessarily prevent them from breaking bones. These results come from the U.S. Preventive Services Task Force report published in the Annals of Internal Medicine.

The outlook is different for women who are over the age of 65, are deficient in vitamin D, or have a history of falls or osteoporosis. For them, the Institute of Medicine says, vitamin D supplements prescribed by a doctor are beneficial.

One risk of getting too much vitamin D is that in healthy people, vitamin D blood levels higher than 100 nanograms per milliliter (ng/mL) can trigger extra calcium absorption — and lead to kidney stones, notes the Cleveland Clinic. And a February 2013 report by the U.S. Preventive Services Task Force found that postmenopausal women who took daily vitamin D and calcium supplements had a 17 percent increased risk of kidney stones compared to women who took a placebo.

To achieve vitamin D recommendations established by the Institute of Medicine – 600 IU per day for people 1 to 70 years old and 800 IU per day for individuals 71 or older – include whole foods such as salmon, tuna, milk, mushrooms, and fortified cereals in your daily diet.

2. Calcium: The Excess Settles in Your Arteries

Calcium is central for strong bones and a healthy heart, but too much is not a good thing. “Get calcium from your diet if you can,” says Dr. Millstine. Research shows that calcium is better absorbed through food. The National Institutes of Health (NIH) recommends 1,000 milligrams (mg) of calcium a day for women aged 19 to 50 and 1,200 mg a day for women 51 and older. Yogurt contains about 207 mg of calcium in 4 ounces, one-fifth of the daily recommendations. Other good calcium sources include milk, cheese, and fortified cereal and juices.

Calcium deficiency or hypocalcemia may be detected by routine blood tests. If you have low calcium blood levels, your doctor may prescribe a calcium supplement.

However, an excess of calcium, which is described by the NIH as more than 2,500 mg per day for adults ages 19 to 50, and more than 2,000 mg per day for individuals 51 and over, can lead to problems. According to the Cleveland Clinic, “Researchers believe that without adequate vitamin D to help absorb it, the extra calcium settles in the arteries instead of the bones.”

3. Multivitamins: No Substitute for a Healthy Diet

Many people believe that they do not get enough vitamins and minerals from their diet. However, the jury’s still out on whether these supplements are beneficial.

An October 2011 study published in the Archives of Internal Medicine examined data from nearly 40,000 women over 19 years. Surprisingly, researchers found that, on average, women who took supplements had an increased risk of dying compared with women who didn’t take supplements. Multivitamins did little or nothing to protect against common cancers, cardiovascular disease, or death.

However, more recent research has found benefits to taking multivitamins. In a January 2015 study in the Journal of Nutrition of more than 8,000 men and women over the age of 40, women who took a multivitamin for three or more years had a lower risk of heart disease.

For women of childbearing age, taking prenatal vitamins with folic acid is recommended to help prevent birth defects. Multivitamins might also be prescribed by your doctor if you have malabsorption syndrome, a condition in which the body does not properly absorb vitamins and minerals.

But for healthy people, Manson notes, “A supplement can never be a substitute for a healthy diet."

4. Fish Oil Supplements: Choose Fish or Flaxseed Instead

Rich in omega-3 fatty acids, fish oil has been touted as a means to reduce heart disease. However, more and more evidence shows that fish oil supplements have questionable heart benefits. A May 2013 study in the New England Journal of Medicine gave 6,000 people at high risk for cardiovascular disease 1,000 mg of omega-3 supplements per day for five years. In the end, however, the high-risk group fared no better in terms of cardiovascular death rates than participants who received a placebo.

Doctors agree that the best way to get your omega-3s is from food. According to the Mayo Clinic, eating fish, which is rich in omega-3 fatty acids, seems to provide more benefit to your heart health than taking supplements. And the American Heart Association (AHA) Dietary Guidelines recommend including two servings of fish per week in your diet.

For people with heart disease, the AHA recommends having 1 gram (gm) of omega-3s per day. If you have high triglycerides, the AHA recommends 2 gm to 4 gm in the form of doctor-prescribed supplements. Other sources of omega-3s beside fatty fish include flaxseeds, walnuts, and avocados.