The controversy about whether megadoses of vitamin C are healthful began in 1970 when Noble Prize winner Linus Pauling published his controversial book Vitamin C and the Common Cold.  This controversy is still very alive today.
However, for most purposes, vitamin C is nontoxic.  Gram amounts are widely used as the treatment for several conditions such as the common cold, cancer, schizophrenia, drug addiction, and improvement of immune resistance.  Vitamin C is not stored appreciably in the body and excess amounts are eliminated rapidly through the urine.  Amounts over 10 grams per day are associated with some side effects, though none are serious.

Most of the claimed clinical adverse effects of high doses are anecdotal and result from uncontrolled studies.  It has been claimed that ingestion of excessive quantities can contribute to kidney stone formation, "rebound" scurvy, vitamin B₁₂ destruction, mutagenicity, impaired copper utilization, transient diarrhea, and laboratory testing errors.  Only the latter two effects appear to be supported by some published literature.

High intakes of ascorbic acid were postulated to contribute to kidney stone formation because oxalate, a component of stones, is produced during catabolism of the vitamin.  However, the oxalate produced from vitamin C is insignificant in comparison with oxalate from dietary sources.  This is a rare case and only people with kidney disease should be concerned.

The suggestion of "rebound" scurvy, the appearance of scorbutic symptoms following the withdrawal of high dose vitamin C intake, has no experimental evidence for such an effect in humans.  Guinea pig studies in this area have been negative or inconclusive.

The theories that large amounts of ascorbic acid destroyed vitamin B₁₂ and that vitamin C is mutagenic were shown to be a result of error in experimental testing.  Controlled studies have also shown no support for the claim that vitamin C impairs copper absorption.

Diarrhea is the only major side effect of large doses for the otherwise healthy person.  Diarrhea is usually the first sign that the body's tissue fluids have been saturated with ascorbic acid.  These disturbances are due to the laxative action of vitamin C.  Most people will not experience this effect with under 5-10 grams of vitamin C per day.

It also have been proven that megadoses of vitamin C may cause interference with the clinical analysis of blood and urine.  Excess intake gives false positive results for tests of diabetes and interferes with tests for hemoglobin.

Other minor side effects include nausea, stomach cramping, and vomiting.

For high exposure levels, be aware that chewable tablets are full of sugar and the acidity of vitamin C is hard on tooth enamel.