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.

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.

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