Various dosages of vitamin C have been used depending on the purpose:
— Treating common colds: One to three grams (g) a day
— Preventing common colds: 600-1,000 milligrams (mg) a day
— Preventing sunburn: Two grams of vitamin C together with vitamin E (1,000 international units [IU])
— Preventing hardening of the artery: Slow-release vitamin C 250 mg plus 136 IU of vitamin E twice a day
Since the adverse effects of taking vitamin C are directly related to the dose, it is important for you to know what is the tolerable upper intake level is for your age group. And that’s easy: anybody over the age of 19 shouldn’t take more than 2,000 mg of vitamin C a day.
Common adverse effects of too high a dose include nausea, vomiting, heartburn, abdominal pain, diarrhea, flushing, headache, insomnia, fatigue, and bowel obstruction. Any dose higher than the tolerable upper intake of 2,000 mg a day could lead to these gastrointestinal symptoms.
Another side effect from an overdose of vitamin C is kidney stones. It has been estimated that those individuals taking over one gram per day of vitamin C increase their risk of kidney stones by 40%. Moreover, large doses of vitamin C have been associated with deep vein thrombosis. In postmenopausal women with diabetes, supplemental vitamin C in doses greater than 300 mg a day is associated with an increased risk of cardiac death. Also, there is preliminary evidence to suggest that supplemental intake of vitamin C at 500 mg a day for 18 months can cause a 2.5-fold increased rate of arterial wall thickening in the carotid artery (main blood supply to the brain) in non-smoking men and a five-fold increase in the men who smoke.
Several drugs can lower the blood levels of vitamin C and therefore require an increase in the dosage (speak to your doctor). They are the birth control pill and aspirin. If you take two aspirin tablets four times a day for seven days, vitamin-C levels in your white blood cells are lowered by 50%. Also, large doses of vitamin C may interfere with the action of a common blood thinner, warfarin, requiring an increase in warfarin dose. If you are on blood thinners, it is advisable to limit your vitamin C intake to no more than one gram a day and be sure to let your doctor know about this.
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