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Some of the vitamins and supplements may interact with oral contraceptives (OCs) and thus reduce the effectiveness of desired birth control.

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Here are some of the examples:
– St. John’s Wort speeds up metabolism of oral contraceptives resulting in decreased amount of hormones in OCs which are absorbed.
– Excessive consumption of grapefruit or drinking grapefruit juice could impact the amount of many medications, including OCs that the body absorbs.
– Activated charcoal may bind to OC hormones in the gut and thus render them ineffective.
– Soy products, such as tempeh and tofu contain isoflavones which are a type of phytoestrogen. Phytoestrogens can potentially impact the estrogen in birth control. However, more research is needed to determine the effects of phytoestrogens on birth control.
When on birth control, it is strongly advised to discuss one’s goals of taking supplements, including vitamins and minerals, with the doctor prescribing the specific OCs.

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Magnesium, calcium and multivitamins:
If you take magnesium, it is not recommended to take it at the same time as your multivitamins. Magnesium may interfere with the absorption of other minerals found in the multivitamins, like iron and zinc. Additionally, it is not recommended to taking calcium, magnesium or zinc together, because these compounds will “compete for absorption.” Calcium may also prevent the absorption of iron. Also, while taking calcium and magnesium may help to prevent development of osteoporosis, in order to maximize the benefits, it can be recommended to take them at least some two hours apart.

Vitamins D, E and K:
Some studies have shown that absorption of Vitamin K may be reduced when other fat-soluble vitamins such as Vitamin E and Vitamin D are taken together. Similar advice can be given – take these vitamins at least two hours apart to maximize their absorption.

Additional list of incompatibilities:
– Fish Oil & Gingko Biloba – both have blood-thinning potential, therefore the effect can be compounded and potentially result in uncontrollable bleeding or inability to clot.
– Copper and zinc – If one must take both, take them at least two hours apart.
– Iron and Green tea – Skip green tea on days when you’re taking your iron supplements.
– Vitamin C and B12 – some studies have shown that Vitamin C could break down Vitamin B12 in one’s digestive tract, thus reducing B12 absorption. Recommendation – wait at least two hours before taking them.

Conclusion:
Taking supplements properly, can a very daunting task.