And in some cases, they can even be dangerous. Here's why you should think twice before popping a pill.

Credit: Getty Images

We love Gwyneth Paltrow, and are always willing to try her favorite recipes, beauty products, and workouts (like us, she's a Tracy Anderson devotee). But there's no denying that the Goop founder has doled out some questionable health advice over the years. (Who can forget vaginal steaming, bee sting therapy, or her recommendation that you put a $66 jade egg in your vagina?)

So we were skeptical when GP revealed her latest wellness venture: a line of supplements sold on the Goop Shop. There are four types, tailored for different needs. The Mother Load is designed to replenish new moms. Balls in the Air is formulated for women who function at an "intense pace." High School Genes is marketed as a metabolism-booster. And Why Am I So Effing Tired? is supposed to combat fatigue. Each costs $90 for a one-month supply.

Supplements are extremely popular (more than half of American adults take them) and Gwyneth certainly isn't the only one shilling for them. But experts stress that with a few exceptions—such as pregnant women, for example, or those with certain medical conditions—most people probably don't need a supplement.

"Dietary supplements are not necessary nor proven helpful for the average person," says David S. Seres, MD, director of medical nutrition and associate professor of medicine at Columbia Medical Center. "For the most part, [they're] an unnecessary expense and may, in some instances, be dangerous."

Here are three reasons to talk to your doctor before you start popping a new pill—whether it's from Gwyneth's line or your local pharmacy.

You could overdo it

Yes, vitamins and minerals are good for you, and essential to your body's functions. But more isn't always better. "Taking a higher dose of something that the body needs—despite misleading claims such as 'supports heart health'—does not ensure that it will be beneficial, or even safe," says Dr. Seres.

Supplements can contain concentrated doses of different vitamins and minerals, which makes it difficult to keep track of how much you're ingesting. And overdoing it on certain nutrients can lead to nasty side effects. Too much magnesium, for example, can result in symptoms like diarrhea, nausea, and abdominal cramping; iron overload could damage your internal organs; hypercalcemia (too much calcium) can cause vomiting, confusion, and fatigue; and too much zinc can lead to nausea, diarrhea, and headaches.

Specific ingredients in supplements (such as kava or green tea extra powder) can also have negative side effects that range from irritating to downright risky. Last year, Consumer Reports released a list of 15 supplement ingredients that have been linked to serious health problems, including cancer, organ damage, and cardiac arrest.

Supplements may interact with your medication

Supplements get particularly tricky when you take them along with prescription drugs. This is especially true for people with heart problems; herbal remedies like echinacea, gingko, green tea extract, and ginger can interfere with medications such as blood thinners, anti-platelet drugs, calcium channel blockers, aspirin, and NSAIDs. And the National Institutes of Health warns that vitamin K can reduce the effectiveness of the blood thinner Coumadin.

The NIH also cautions that St. John's wort can interact with a variety of drugs, including birth control pills and antidepressants, while vitamin C and E supplements may make some types of chemotherapy less effective.

Risky interactions can occur when you take more than one supplement as well. For example, fish oil plus an herb that slows blood clotting (such as Ginkgo bilboa) can actually cause bleeding. And taking melatonin along with another supplement that can cause drowsiness (such as St. John's wort or valerian) can amplify its effects.

A healthy diet can provide all the nutrients you need

Your doc may recommend a supplement if you're on a diet that eliminates an entire food group, or if you have a true nutrient deficiency or suffer from a medical condition like anemia or kidney failure. But most people are able to get the nutrients they need through a healthy diet that includes plenty of fresh produce, lean proteins, and healthy fats, experts say.

"Supplements are a complement to good nutrition, not a stand-in or excuse for poor eating habits," says Eliza Whetzel Savage, RD, a clinical nutritionist at Middleberg Nutrition in New York City. If you do decide to take a supplement, you really should consult your doctor, says Savage. It's also smart to read the product's ingredient list, look for a USP verified mark, and purchase it from a trusted source.

Bottom line: "Stick to a healthy diet first," Savage urges. "And if you seek supplementation, see a specialist in the field."