Vitamin B12, or cabalamin, is water-soluble, meaning the renal system filters the vitamin from blood along with water, so it is difficult to build excess. If you do build an excess of the vitamin, it is not necessarily negative: The body can store enough B12 in the liver that those with maximum stores can go years without ingesting it at all. It still is possible to greatly exceed even the amount that the body can store with vitamin supplements, given that the body uses vitamin B12 in very small amounts usually measured in micrograms.
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Reduce the amount of animal-derived foods that appear in your diet, as they contain vitamin B12. These include meat, poultry, dairy and shellfish.
Drink more water to help flush your body of water-soluble elements in the blood, which includes vitamin B12.
Exercise, since doing so can increase your body's use of water, which will increase necessary water consumption and renal activity. When hydrating, avoid energy and sport drinks that include vitamins, which could include B-complex vitamins like B12. Water is the best option.
Ingest mild diuretics like caffeine, a typical component of coffee. If you choose to drink coffee, avoid using dairy products as additives, as they likely contain vitamin B12. Mild diuretics also can increase renal activity, filtering more B12 to reduce excess. However, it is inadvisable to use strong diuretics unless suggested by your doctor, as they can be harmful if misused.