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Acute toxicity is unlikely after ingestion of vitamin products that do not contain iron (for situations in which iron is present, see Iron). Vitamins A and D may cause toxicity, but usually only after chronic use. Serious toxicity has been reported in individuals attempting to mask urine drug screens by ingesting large quantities of niacin.

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  1. Mechanism of toxicity

    1. Vitamin A. The mechanism by which excessive amounts of vitamin A produce increased intracranial pressure is not known.

    2. Vitamin C. Chronic excessive use and large IV doses can produce increased levels of the metabolite oxalic acid. Urinary acidification promotes calcium oxalate crystal formation, which can result in nephropathy or acute renal failure.

    3. Vitamin D. Chronic ingestion of excessive amounts of vitamin D enhances calcium absorption and produces hypercalcemia.

    4. Niacin. The most common adverse effects of niacin are cutaneous flushing and pruritus mediated by prostaglandin release.

    5. Pyridoxine. Chronic overdose may alter neuronal conduction, resulting in paresthesias and muscular incoordination.

  2. Toxic dose

    1. Vitamin A. Acute ingestion of more than 12,000 IU/kg is considered toxic. Chronic ingestion of more than 25,000 IU/d for 2–3 weeks may produce toxicity.

    2. Vitamin C. Acute intravenous doses of more than 1.5 g and chronic ingestion of more than 4 g/d have produced nephropathy.

    3. Vitamin D. Acute ingestion is highly unlikely to produce toxicity. In children, chronic ingestion of more than 5000 IU/d for several weeks may result in toxicity (adults >25,000 IU/d).

    4. Niacin. Acute ingestion of more than 100 mg may cause a dermal flushing reaction. Immediate-release products are more likely to cause flushing than are the timed-release preparations. Ingestion of 2.5 g produced nausea, vomiting, dizziness, hypoglycemia followed by hyperglycemia, and coagulopathy.

    5. Pyridoxine. Chronic ingestion of 2–5 g/d for several months has resulted in neuropathy.

  3. Clinical presentation. Most acute overdoses of multivitamins are associated with nausea, vomiting, and diarrhea.

    1. Chronic vitamin A toxicity is characterized by dry, peeling skin; alopecia; and signs of increased intracranial pressure (headache, altered mental status, and blurred vision [pseudotumor cerebri]). Bulging fontanelles have been described in infants. Liver injury may cause jaundice and ascites.

    2. Vitamin C. Calcium oxalate crystals may cause acute renal failure or chronic nephropathy.

    3. Chronic excessive use of vitamin D is associated with hypercalcemia, producing weakness, altered mental status, GI upset, renal tubular injury, and occasionally cardiac arrhythmias.

    4. Chronic excessive use of vitamin E can cause nausea, headaches, and weakness.

    5. Vitamin K can cause hemolysis in newborns (particularly if they are G6PD deficient).

    6. Acute ingestion of niacin, but not niacinamide (nicotinamide), may produce unpleasant, dramatic cutaneous flushing and pruritus that may last for a few hours. Intentional ingestion of large amounts in an attempt to produce a negative urine drug screen has caused nausea, vomiting, abdominal pain, palpitations, dizziness, and hypoglycemia, followed by persistent hyperglycemia, anion gap metabolic acidosis, hypotension, and coagulopathy. Chronic excessive use (particularly of the sustained-release form) has been associated with hepatitis.

    7. Chronic excessive pyridoxine use may result in peripheral neuropathy.

    8. Large doses of B vitamins may intensify the yellow ...

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