I find this question difficult to answer because whether or not vitamins and supplements will help with an illness will depend on what illness it is.
Generally I think the concept of taking vitamins and supplements is not as useful as some people believe. If your symptoms are caused by a deficiency in <whatever> then taking vitamins and/or supplements to make it for it will obviously help. For instance, if your iron levels are low and are causing you fatigue, then taking iron supplements will boost your iron levels and therefore will help alleviate your fatigue.
Otherwise, if you're megadosing yourself on vitamins and supplements in the belief that it'll benefit you in some way, then AFAIK it's not that useful. Your body would just dump the excess vitamins and/or supplements out. You might as well just eat a healthy, balanced diet instead.
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