While it's impossible to say exactly how long a multivitamin is effective, understanding how different vitamins are absorbed and stored in the body can give you some idea. Generally, water soluble vitamins (B vitamins and vitamin C) aren't stored in the body. Instead, they are absorbed based on need with the remaining excesses being excreted in the urine. This is why it's important to get these vitamins on a regular basis. On the other hand, fat soluble vitamins (A, D, E and K) need fat present to be absorbed and are stored in the body for later use (fat tissue and liver).
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