Megavitamin therapy involves taking doses of vitamins far larger than are considered necessary for general health. The megadose approach has been used for conditions as diverse as the common cold and cancer, but there are drawbacks.
Possible Adverse Effects
Vitamin A (retinol) Large amounts of retinol can cause liver and bone damage. Since retinol is fat soluble, it can build up in the body. Women who are pregnant or planning to be should not take vitamin A supplements because of the risk of birth defects.
Thiamin(B1) More than 3g a day is toxic to adults causing headache, irritability, insomnia, rapid pulse, weakness, contact dermatitis, pruritis, and even death.
Niacin(B3) Very high doses of nicotinic acid, 3-6g a day, can damage the liver. Around 200mg a day can cause flushing (although it can be avoided by taking B3 as nicotinamide).
Vitamin B6 Nerve function can be damaged at doses of between 2-7 g a day, and some sensory loss is reported in some people between 50-500mg a day with symptoms disappearing after withdrawal of supplements.” High intakes in pregnancy may be risky.
Vitamin C May cause kidney stones in a small group of people who produce too much oxalate in response to high doses.
Vitamin D Infants are most at risk at doses over 50ug a day from calcium deposits in the arteries and excessive calcification of bones and internal organs.
Magnesium High doses between 3-5g a day over a prolonged period can be fatal.
Phosphorus Maximum daily intake is 70mg per kg of body weight or about 4.5g for a 65kg man.
Potassium Intakes above 17.6g a day could be toxic.
Iron Poisoning occurs in children above 20mg per kg body weight a day with between 200-300mg per kg body weight being a lethal dose.
Zinc Poisoning can occur at daily intakes of 2g or more. Long-term exposure to 75-80mg a day can leao to anemia. As little as 50mg a day interferes with the metabolism of iron and copper.
Copper High intakes are toxic but how much is not known.
Selenium Serious problems occur at intakes above 750ug a day. In the absence of more details, a safe intake is set at 450ug a day for adult males.
Iodine High intakes can cause goiter and hyperthyroidism. Upper limit on intakes is 17ug per kg body weight a day.
Cobalt Doses of 29.5mg a day have produced serious toxic effects.
Germanium Doses of 50-250mg a day for between 4 and 18 months can cause serious harm and even death.