: Why do vitamin supplements often have quantities way in excess of RDAs? When I have a cold or otherwise feeling ill, I like to take vitamin C supplements. I've noticed that they almost invariably have amounts way above the

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I don't know why your taking vitamin C supplements when you are ill. Vitamin supplements are usually prescribed when the vitamins in the said person is known to be deficient. They are not prescribed for illness and shouldn't be taken as such as over-dosage causes hyper-vitaminosis or vitamin toxicity.
Having said that, I think you meant to say that the RDA is 90mg(as 90g is way too high). I'm aware that this value is different depending on the country as well as the type of person.
The reason why vitamin supplements are available in high doses is because these are meant to be used for deficiency syndromes(sometimes they're used as placebos as well). They are meant to replenish the lost stores of vitamins in the body. Also note that 1000mg of oral vitamin C does not mean that your body will absorb 100% of it. Infact in 1000mg of oral vitamin C, only about 50% is absorbed by the body. This is what is referred to as "bio-availability." This is because a series of metabolic activities take place in the body, and about 50% is absorbed and 50% is excreted [please note that bioavailability of different drugs will be different].
There is also no point in making vitamin supplements to contain the exact RDA as required as
(a) You would have consumed the RDA equivalent if you are eating a balanced diet
(b) Hence there is no need to create a supplement as such
(c) For 90mg of vitamin C to be absorbed by the body, the dosage of the supplement should around 125mg(here at lower doses bioavailability of vitamin C is around 70-80%). This does not help people will deficiency syndromes