Take your vitamins. For many us, it’s a mantra that has been hammered into our brains as early as we can remember. And it worked: About half of all Americans take vitamin supplements, fueling a $12 billion industry, according to a survey by the Gallup Poll. But the health landscape has changed since we first learned to supplement our diets with Flintstones chewables. A slew of studies has sparked debates about the benefits of daily supplemental vitamins and minerals, and raised concerns about taking too many.

“Vitamins are not inert,” Dr. Eric Klein, a prostate cancer expert and researcher at the Cleveland Clinic told the The New York Times. “They are biologically active agents. We have to think of them in the same way as drugs. If you take too high a dose of them, they cause side effects.”

In a study published in the Journal of the American College of Cardiology, researchers at St. Michael’s Hospital and the University of Toronto found that some common supplements didn’t show significant long-term benefits, while others—particularly antioxidants like Niacin—raised health concerns when taken in higher doses.

“These findings suggest that people should be conscious of the supplements they’re taking and ensure they’re applicable to the specific vitamin or mineral deficiencies they have been advised of by their healthcare provider,” Dr. David Jenkins, the lead author of the study, told Business Insider.

Much of the food we eat every day already delivers a healthy dose of vitamins to our systems. Add on additional supplements—particularly in larger doses— and it could be too much of a good thing.

That doesn’t mean you should halt your vitamin regimen, particularly if it’s doctor recommended. But if you’ve been winging it, or megadosing on your own accord, it’s important to know the facts—specifically how much is too much. Here’s a breakdown of some popular supplements and recommended dosages for each. That said, everyone’s body is different—so check with your physician before you deep-dive into a vitamin regimen.

Multivitamins

These honking pills have sparked plenty of debate in recent years. While the consensus among doctors is that a standard one-a-day pill taken as instructed (one a day!) is safe and potentially helpful in rounding out your diet, it shouldn’t be overused or taken lightly. The National Institutes of Health warns that “individuals who take MVMs and other supplements and who eat fortified foods and beverages might consume some nutrients at levels exceeding the UL (upper tolerable limit), increasing the possibility of adverse effects.” Taking MVMs with additional supplements can also overload your system or create unwanted interactions. Dr. Mehmet Oz advises everyone to “read the labels” on your multivitamins—and to avoid megadose multis. “Look for a multi with no more than 3,500 IU of vitamin A and no more than 30 IU of vitamin E,” Oz states in AARP. “Medical research supports these values as safe doses that can compensate for dietary insufficiency.” He also suggests splitting the pill—taking half in the morning and a half at night so that the supply is distributed throughout the day.

Vitamin D

Vitamin D helps your body absorb calcium, which promotes healthy bones. Aging and lack of sunlight may cause bone loss, which is why doctors have long suggested people over 70, those with minimal sun exposure, as well as patients with diabetes, high blood pressure, heart disease or other risk factors, supplement their diets with Vitamin D. A recent study published in the American Journal of Clinical Nutrition, suggested that taking it in high dosages showed no added benefits. So how much should you take? The Mayo Clinic recommends that adults get at least the RDA of 600 IU (International Units). However, they claim, “1,000 to 2,000 IU per day of vitamin D from a supplement is generally safe.” It’s best to have your doctor check your blood levels for Vitamin D—they should tell you whether you need to supplement your intake or whether your diet alone is enough.

Omega-3 Fatty Acid

The fatty acids found in most fish have been linked to lowered risk of heart disease. Eating foods rich in Omega-3 (think salmon, mackerel and sardines) twice a week is beneficial particularly for those at risk. The American Heart Association suggests people with high triglycerides (blood fats), supplement their diet with “2 to 4 grams of EPA + DHA per day, in the form of capsules and under a physician’s care.”

Vitamin C and Zinc

If you feel a cold coming on, you may immediately reach for these two supplements. But beware: large doses of C—2,000 milligrams or more, can raise the risk of kidney stones, and cause nausea, headaches and other unsavory symptoms—and several studies suggest the vitamin isn’t consistently effective. If you want to give it a shot, the safe dosage for C is 65 to 90 mg. Considering how much C is in our daily diets, it’s best not to overdo it. Zinc, meanwhile, has been proven effective in fighting colds, but again, use with caution. The recommended dosage is 8mg for women and 11mg for men.

When it comes to your health, the first line of defense starts with your diet. “Nutritionists recommend food first because foods provide a variety of vitamins and minerals and also dietary factors that are not found in a vitamin or mineral supplement,” states Penny Kris-Etherton, Ph.D., R.D., and Distinguished Professor of Nutrition at the Pennsylvania State University’s College of Health and Human Development, according to the American Heart Association. She adds that “a supplement is OK if nutrient needs are not being met by a healthy food-based diet.” Just make sure you don’t overdo it.