Gruns Review 2026: Legit or Overhyped?

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.

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.

Serious risks

• No adverse effects have been reported from high thiamin intakes, but the FNB noted that excessive intakes could still have adverse effects.

Avoid or ask a doctor first if

• People who take certain medications regularly should talk with their health care providers about their thiamin status.

Medication interactions

• Thiamin isn't known to interact with any medications, but certain medications can have an adverse effect on thiamin levels.

Serious risks

• Food intakes many times the RDA show no observable toxicity; the source suggests limited solubility and absorption may explain this.
• Limited data on adverse effects don't mean high intakes are harmless, and the FNB urges caution with excessive amounts.

Medication interactions

• Riboflavin is not known to have clinically relevant interactions with medications.

Serious risks

• High biotin intakes, possibly even intakes above the AI, may pose a health risk tied to lab testing.
• Biotin beyond recommended intakes can cause clinically significant falsely high or falsely low lab results, depending on the test.
• In a small study of six healthy adults taking 10 mg/day for 1 week, biotin falsely lowered N-terminal pro-brain natriuretic peptide, which could result in a failure to identify congestive heart failure. Thyroid stimulating hormone was also falsely lowered.
• Case reports describe findings falsely indicating Graves' disease and severe hyperthyroidism in patients taking 10–300 mg biotin per day.
• A single 10 mg dose of biotin has interfered with thyroid function tests given within 24 hours.
• Per FDA, a patient with a high intake of supplemental biotin died after a troponin test gave a falsely low result because of biotin interference.

Medication interactions

• Biotin can interact with certain medications, and some medications can have an adverse effect on biotin levels.

Possible side effects

• Some people taking large doses of pantothenic acid supplements develop mild diarrhea and gastrointestinal distress. The cause is not known.

Serious risks

• The FNB found no reports of pantothenic acid toxicity in humans at high intakes.

Medication interactions

• Pantothenic acid is not known to have clinically relevant interactions with medications.

Possible side effects

• High iodine intakes can cause goiter, elevated TSH levels, and hypothyroidism, some of the same symptoms as iodine deficiency.

Serious risks

• Iodine-induced hyperthyroidism can result from high iodine intakes, including when iodine is given to treat iodine deficiency.
• Studies have shown that excessive iodine intakes cause thyroiditis and thyroid papillary cancer.
• Acute iodine poisoning is rare and is usually caused by doses of many grams. Symptoms include burning of the mouth, throat, and stomach, fever, abdominal pain, nausea, vomiting, diarrhea, weak pulse, and coma.
• Long-term intakes above the UL increase the risk of adverse health effects.

Avoid or ask a doctor first if

• People with autoimmune thyroid disease or iodine deficiency may have adverse effects at iodine intakes considered safe for the general population.
• The ULs do not apply to people receiving iodine for medical treatment, who should be under a physician's care.

Medication interactions

• Iodine supplements have the potential to interact with several types of medications. People taking these medications on a regular basis should discuss their iodine intakes with their health care providers.

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 copper exposure over time can cause gastrointestinal symptoms.

Serious risks

• Chronic exposure to high levels of copper can result in liver damage.
• Copper toxicity has been reported in people who drink water with high copper levels, for example from stagnant water in copper-containing pipes and fixtures.
• People with Wilson's disease can develop neurologic and liver damage that can result in cirrhosis.

Avoid or ask a doctor first if

• People with Wilson's disease have a high risk of copper toxicity.

Medication interactions

• Copper is not known to have any clinically relevant interactions with medications.

Serious risks

• Manganese toxicity mainly affects the central nervous system and can cause tremors, muscle spasms, tinnitus, hearing loss, and unsteadiness on one's feet.
• People who drink water with high manganese levels have also developed manganese toxicity.
• Workers such as welders and miners who chronically inhaled manganese dust have developed manganese toxicity.
• Toxicity signs and symptoms can progress to Parkinson's-like neuromotor impairments, including changes in gait and balance, tremor, and rigidity.

Avoid or ask a doctor first if

• People with chronic liver disease eliminate manganese less well in bile and are more susceptible to its neurotoxicity and other adverse effects of excess intake.
• Iron deficiency increases manganese absorption and can worsen symptoms of manganese toxicity.

Medication interactions

• Manganese is not known to have any clinically relevant interactions with medications.

Serious risks

• Isolated case reports link chromium supplements to several problems, including liver dysfunction and renal failure.
• The FNB said caution may be warranted because data are limited, and high intakes could have adverse effects.

Avoid or ask a doctor first if

• People with renal or liver disease might be susceptible to adverse effects from high chromium intakes.

Medication interactions

• Several types of medications have the potential to interact with chromium supplements.
• People taking medications regularly should talk with their health care providers about their chromium intakes.

Serious risks

• Acute molybdenum toxicity is rare but can occur with industrial mining and metalworking exposure.
• In one study of high dietary molybdenum intake in an area of Armenia, affected people had achy joints, gout-like symptoms, and abnormally high blood uric acid.

Avoid or ask a doctor first if

• Infants up to 12 months should get molybdenum only from breast milk, formula, and food.

Medication interactions

• The source says molybdenum has no known, clinically relevant, interactions with medications.

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.