Ultimate Multivitamin Review 2026: Worth the Price?

Serious risks

• The Food and Nutrition Board set no upper limit for B12 because its potential for toxicity is low.

Avoid or ask a doctor first if

• People who regularly take certain medications are told to talk with their health care providers about their B12 status.

Medication interactions

• B12 supplements may interact with some medications. Gastric acid inhibitors and metformin may affect B12 levels.
• Several types of medications might adversely affect B12 levels.

Serious risks

• The Food and Nutrition Board reported no adverse effects from vitamin K in food or supplements in humans or animals.

Avoid or ask a doctor first if

• People taking medications regularly should talk with their health care providers about their vitamin K status.

Medication interactions

• Vitamin K can interact with warfarin and other anticoagulants.
• Antibiotics, bile acid sequestrants, and orlistat can have adverse effects on vitamin K levels.

Possible side effects

• The most common complaints are diarrhea, nausea, abdominal cramps, and other digestive upset.

Serious risks

• High intakes may raise urinary oxalate and uric acid, which could contribute to kidney stones, especially in people with renal disorders.
• The best evidence that vitamin C contributes to kidney stones is in patients who already have hyperoxaluria.
• In people with hereditary hemochromatosis, long-term high doses could worsen iron overload and cause tissue damage.
• A few in vitro studies suggested that supplemental oral vitamin C, acting as a pro-oxidant, could cause chromosomal or DNA damage and possibly contribute to cancer.
• In postmenopausal women with diabetes in the Iowa Women's Health Study, supplemental vitamin C of at least 300 mg/day was significantly associated with a higher risk of dying from CVD.

Avoid or ask a doctor first if

• People with hereditary hemochromatosis face a concern about iron overload with chronic high doses.
• The upper limits do not apply to people receiving vitamin C as medical treatment, who should be under a physician's care.
• Infants should get vitamin C only from formula and food.

Medication interactions

• Vitamin C supplements may interact with radiation therapy and with medications, including chemotherapy drugs and statins.
• People taking these medications regularly should talk with their health care providers about vitamin C intake.

Possible side effects

• High zinc intakes can cause nausea, dizziness, headaches, gastric distress, vomiting, and loss of appetite.

Serious risks

• Doses of 50 mg or more taken for weeks can interfere with copper absorption, reduce immune function, and lower HDL cholesterol.
• Very high supplemental doses (142 mg/day) might interfere with magnesium absorption and disrupt magnesium balance.
• According to a few reports, overuse of zinc-containing denture adhesive creams can lead to neurological symptoms and anemia.

Avoid or ask a doctor first if

• The upper limits don't apply to people receiving zinc for medical treatment, who should be under a physician's care.
• Zinc-free denture adhesive formulations are available to prevent the effects linked to zinc-containing creams.

Medication interactions

• Zinc may interact with quinolone antibiotics, tetracycline antibiotics, and penicillamine.
• Some diuretics can lower serum zinc by increasing zinc excretion in urine.
• People taking medications regularly should discuss their zinc status with their health care providers.

Possible side effects

• Early signs of too much selenium are a garlic odor on the breath and a metallic taste in the mouth.
• Hair loss and nail brittleness or loss are the most common clinical signs of chronically high selenium intake.
• Other signs of chronically high intake include skin rash, nausea, diarrhea, fatigue, irritability, and nervous system abnormalities.

Serious risks

• Acute selenium toxicity can cause severe gastrointestinal and neurological symptoms, respiratory distress syndrome, heart attack, kidney failure, cardiac failure, and, in rare cases, death.
• Acute toxicity has resulted from misformulated over-the-counter products containing very large amounts of selenium. In 2008, 201 people had severe adverse reactions to a liquid supplement with 200 times the labeled amount.
• Brazil nuts contain very high amounts of selenium and could cause selenium toxicity if eaten regularly.

Avoid or ask a doctor first if

• The upper limits do not apply to people taking selenium under a physician's care.
• People who take medications regularly should discuss their selenium status with their health care providers.

Medication interactions

• Selenium supplements may interact with medications, and some medications, such as cisplatin, can affect selenium levels in the body.

Possible side effects

• High-dose iron supplements can cause stomach upset, constipation, nausea, abdominal pain, vomiting, and diarrhea.
• Supplements with 25 mg iron or more can reduce zinc absorption and plasma zinc concentrations.

Serious risks

• Large acute intakes from supplements or medicines can lead to corrosive necrosis of the intestine, which might lead to fluid and blood loss, shock, tissue damage, and organ failure.
• In severe cases, iron overdoses can lead to multisystem organ failure, coma, convulsions, and even death.
• Case reports suggest some people develop more serious gastrointestinal effects, including gastritis and gastric lesions.
• At least 43 U.S. children died between 1983 and 2000 from ingesting high-dose iron supplements.
• Without treatment, people with hereditary hemochromatosis typically develop signs of iron toxicity by their 30s.
• Iron toxicity effects in hemochromatosis can include liver cirrhosis, hepatocellular carcinoma, heart disease, and impaired pancreatic function.

Avoid or ask a doctor first if

• People with hemochromatosis: treatment guidance includes avoiding iron and vitamin C supplements.
• The required FDA label on solid iron supplements says accidental overdose is a leading cause of fatal poisoning in children under 6, and to keep the product out of children's reach.

Medication interactions

• Iron supplements may interact with levodopa and levothyroxine.
• Proton pump inhibitors can potentially reduce iron absorption.