There are more than 100,000 supplements on the US market – capsules, powders, tablets and gummies sold to improve or maintain health. Supplements can contain vitamins, minerals, botanicals and amino acids on their own or in various combinations.

The consumption of these products is surging. But it’s a common misunderstanding that these products are entirely safe, says Dr Pieter Cohen, an internist and associate professor at Harvard Medical School. Excessive amounts of nutrients can cause health problems, so it’s critical to know if you’re using the correct dosage of high-quality products.

Here’s what experts say about navigating the world of supplements, and what happens if you take too much of some common ones.

Choose supplements and dosage carefully

Ideally, the nutrients we need come from the food we eat, says Dr Denise Millstine, director of integrative medicine at the Mayo Clinic Arizona. But that’s not always possible for various reasons, like a lack of access to nutritious foods or the effects of certain medications.

“Some people may need a supplement; some people need to be careful that they’re not getting too much of a supplement,” says Joan Blake, a registered dietitian and clinical instructor at Boston University. “It’s all over the place.”

There are so many of these products for sale, and so many variants of what’s on a label, that shopping and selecting even a single supplement can be “dizzying” for consumers, says Millstine.

For some nutrients, there’s a recommended dietary allowance (RDA) – an amount to aim for daily, set in the US by the Food and Nutrition Board of the National Academies of Sciences, Engineering, and Medicine. The RDA can vary by sex, age and medical status. There’s also a tolerable upper intake level (UL), which is the maximum safe daily limit. Consuming more than the UL can cause health problems, depending on the nutrient.

Additionally, in the US, “there’s no vetting of products before they’re sold to ensure that the label is accurate,” says Cohen, whoseresearch suggests contents and quantity labelling are often inaccurate.

There is limited government oversight over vitamins and supplements. The US Food and Drug Administration (FDA) does not approve supplements or their labelling before they are sold. The only way to know if what’s listed on the label is accurate is if the product has been certified by the National Sanitation Foundation (NSF) or United States Pharmacopeia (USP), says Cohen. These are non-profits with “high-quality, third-party certification programs”, he explains. You can use their websites to find verified products.

Check with your doctor before taking any new supplement

Because the recommended amount differs from person to person, it’s critical to speak with a healthcare provider before starting something new. You need to know how the product might interact with anything else you’re consuming, like a medication or another supplement, explains Cohen. Adverse health effects could arise, or medications can become less effective.

Supplements can also affect blood tests, leading to inaccurate results, explains Dr Fernando Carnavali, an associate professor of internal medicine at the Icahn School of Medicine at Mount Sinai. He says it’s important to continue to discuss any vitamin or supplement use with your physician in case of evolving research about potential interactions, benefits or harms.

It’s best to use vitamins and supplements with caution, just as you would with over-the-counter medications like Tylenol, says Millstine. It’s also a good idea to stick to studied doses. She recommends using the National Center for Complementary and Integrative Health website for information on dosing. The US Department of Agriculture also has a tool for calculating the amount of vitamins and minerals you need.

Conversely, do not take dosing advice from the person selling the product, due to a risk of bias, says Millstine.

Carnavali suspects that because vitamins and minerals are perceived as natural, people are less likely to share what they are taking with their medical providers. It’s not uncommon for a patient to come in with questions about their health, only to learn that their vitamin and supplement habits were causing symptoms. “We’re not here to judge – we’re here to guide you,” says Carnavali.

What happens if you take too much?

**Multivitamins: **Multivitamin supplements contain at least three vitamins and one mineral.

There’s no regulation of what nutrients must be included or how much. According to the National Institutes of Health, some of these supplements contain vitamins and minerals in amounts higher than the established UL. Accordingly, there’s a risk of excess nutrients, especially if you’re taking a multivitamin with other supplements.

What harm might occur depends on the excessive nutrient in question. “Multivitamins can be tricky because you need to look at each of the components, and there are just so many of them,” says Millstine.

There are risks associated with certain individuals. For example, the NIH states that smokers and former smokers might want to avoid multivitamins with large amounts of beta-carotene or vitamin A, which are linked to an increased risk of lung cancer in smokers. For people who are pregnant, more than 2,800micrograms (mcg) of vitamin A a day can increase the risk of birth defects.

People who take blood thinners should speak with their doctor before using a multivitamin containing vitamin K, which can decrease the effectiveness of those drugs.

**Vitamin D: **“Vitamin D can become quite toxic when taken excessively,” says Millstine. High levels are associated with harms like nausea, vomiting and excessive urination. Extremely high levels are linked to kidney failure, irregular heartbeat and death. The NIH notes that high levels are “almost always” caused by excessive supplements.

Individual requirements can differ, Millstine says. According to the NIH, the RDA for adults between ages 19 and 70 is around 15mcg, or 600 International Units (IU). The upper limit for the same group is 100mcg, or 4,000 IU.

If your healthcare provider recommends you take vitamin D, it’s advisable to start at the standard dose, then recheck your levels in three months, says Millstine.

**Omega-3 ****fatty ****acids: **Omega-3 fatty acids are found in certain foods, like fatty fish and chia seeds. Supplements like fish oil or fish liver oil contain two types of omega-3 fatty acids: docosahexaenoic acid, or DHA, and eicosapentaenoic acid, or EPA.

Experts haven’t established recommended amounts for these two types, but the FDA recommends limiting the combined total to no more than 5 grams daily from dietary supplements. High doses can increase the risk of bleeding and stroke, according to the Mayo Clinic.

A low-quality omega-3 product comes with more risks of side effects, ranging from heartburn to nausea, explains Millstine. These supplements can contain other elements, like vitamins A and D, and dosage can vary from product to product, so it’s important to check the label.

For instance, if you’re taking a 1,000mg fish oil capsule with a 300mg omega-3 content, it’s worth checking what else is in the product, says Millstine. Non-EPA/DHA elements can cause issues like gastrointestinal symptoms and increased risk of bruising.

Another omega-3 fatty acid, alpha-linolenic acid (ALA), is found in plants. Apart from the food you eat, you can get ALA from flaxseed oil supplements. The recommended amount of ALA for adults is between 1.1 grams to 1.6 grams.

**Calcium: **The typical recommendation for adults is 1,000mg to 1,200mg of calcium daily, divided into no more than around 500mg at a time, says Millstine. The upper limit varies depending on your age and medical status. Calcium can negatively interact with other supplements and medications.

Too much supplemental calcium can cause kidney stones. While research is mixed and more is needed, some studies suggest calcium supplements can increase the risk of heart disease, specifically in postmenopausal women.

According to the National Health Service, more than 1,500mg of calcium a day can lead to stomach pain and diarrhea.

**Vitamin C: **Vitamin C is one of the more overused supplements, because so many people use it to avoid sickness, Blake says. While it may slightly shorten the duration of a cold, research suggests that it won’t prevent a cold.

Extra vitamin C simply exits the body during urination. But chronic excessive use can lead to gastrointestinal issues like nausea and diarrhea, explains Blake. Doses greater than 2,000mg a day can lead to stomach aches, diarrhea and kidney stones.

Millstine advises about 500mg to 1,000mg per day. “The problem with taking too much is it’s just not helping you, so why would you stress your kidneys in that way?” she asks.

**Vitamin B12: **The NIH recommends 2.4mcg of B12 daily for most adults, though personalized doses can vary. It’s available in different types of supplements, many of which have very high doses of B12.

While rare, excessive B12 can be associated with symptoms like anxiety, agitation, flushing and headaches, says Millstine. It can interact with medications like gastric acid inhibitors and metformin. Other forms of vitamin B, like vitamin B6, can be more toxic.

**Magnesium: **Magnesium supplements come in various forms, including magnesium oxide, citrate and chloride. These have different effects: For example, magnesium citrate is used as a laxative, so those with more sensitive stomachs might want to avoid it.

Generally, the RDA for adults is about 300mg to 400mg (depending on age and sex). The UL for magnesium via supplements and medications specifically is 350mg.

While there’s no risk linked to too much magnesium in food, high doses of magnesium might cause nausea, abdominal cramping and diarrhea. Very large doses can be fatal.

People with kidney disease can’t excrete excess magnesium and should not take it unless it’s been discussed with their healthcare team, says Millstine.

**Probiotics: **There is no official recommendation for the use of probiotics by generally healthy individuals. Blake recommends speaking with a registered dietitian nutritionist to determine which strain and dosage is right for you. Possible negative side effects include eczema and inflammatory bowel disease.

If you can tolerate probiotics, excessive use may not be toxic, but there could be other downstream effects.

“Probiotics can limit your ability to get to your natural, healthy microbiome,” says Millstine, because high-dose supplements can hinder the diversity of your gut bacteria. “I caution people to really consider if they need probiotics.”

**Creatine: **Creatine supplements can cause some bloating and GI symptoms, as well as nausea, says Millstine. She recommends sticking to 4 to 5 grams per day.

Research suggests that higher doses of creatine do not offer any advantages. Because there’s mixed evidence of a negative effect on kidneys, people with kidney disease are especially advised to speak with their healthcare provider before trying it.

**Collagen peptides: **Collagen peptides are smaller pieces of collagen and can come in powder and pill form. Collagen peptides can possibly help with joint and skin health, though much more research is needed to support these claims, says Carnavali.

Though there’s no standardized dose, some research shows between 2.5 grams to 15 grams daily is safe. However, research not sponsored by supplement manufacturers is scarce.

Millstine’s primary caution is not to use collagen peptides in place of a more complete protein powder, because they lack essential amino acids needed for overall health.