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Dietary Supplements

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Dietary Supplements
Summary
  • Dietary supplements are generally available without prescription and their supply is not tightly regulated.
  • Dietary supplements should be taken with extreme caution by people who are using prescription medications.
  • If you are taking medication, check with your Doctor to see if there are any possible nutrient interactions that occur as a result of taking the drug.
Description
Traditionally, the term 'dietary supplements' referred to products containing the essential nutrients (e.g. vitamins, minerals and proteins), but the term now includes herbs and other plant-derived substances, amino acids (the building blocks of protein) and a number of other substances. Dietary supplements are generally available without prescription and their supply is not tightly regulated. Drugs are substances that are intended to diagnose, cure, alleviate, treat or prevent disease1 and are subject to very strict regulation and supply.

Dietary supplements are very popular and many people take them in the belief that, because many are natural, they are completely safe. This is not the case. Dietary supplements should be taken with extreme caution by people who are using prescription medications2 and advice should be sought from a Doctor regarding any possible side effects and drug interactions that can arise by combining prescription medications with dietary supplements.

The following is a list of possible vitamin and mineral supplements that may cause problems.

VITAMIN B6 and LEVODOPA - This is a vitamin well known for its ability to help with fluid retention. Unfortunately levodopa, a drug used to treat Parkinson's Disease, can be affected by even small doses of vitamin B6 (5 to 10 mg) and the two should not be taken together.

VITAMIN B6 and PHENYTOIN - Phenytoin (Dilantin) is used for the treatment of seizures. Small doses up to 10 mg a day of this vitamin may be safe, however, after phenytoin is taken, Vitamin B6 can cause the level of this drug in the blood stream to be reduced.

VITAMIN B6 and LIPID LOWERING/CHOLESTEROL LOWERING DRUGS - May increase the risk of rhabdomyolosis and myopathy.3

CYCLOSERINE, HYDRALAZINE, ISONIAZID, PENICILLAMINE - May cause peripheral neuritis by inducing a deficiency in Vitamin B6.4

VITAMIN E and WARFARIN - Vitamin E, particularly in higher doses of more than 400 mg, is used to help with blood circulation. Taking a prescription anticoagulant such as warfarin (e.g. Coumadin) at the same time as Vitamin E, should be avoided otherwise continuous bleeding might result. Both Vitamin E and warfarin are anticoagulants which means that the combined effect of these medications reduces the body's ability to form clots that stop the blood flow from wounds.

VITAMIN E and INSULIN - Vitamin E may reduce the requirement for insulin in diabetics, so insulin levels should be monitored.5

VITAMIN A and ANTICOAGULANT DRUGS - Large doses of vitamin A (greater than 750 to 2500 international units) may reduce prothrombin levels in those on anti-coagulant drugs.6

VITAMIN K and ANTICOAGULANT DRUGS - This vitamin is used to promote clotting of the blood and should not be taken at the same time as an anticoagulant. Vitamin K has an opposite effect to an anticoagulant i.e., it increases the body's ability to form blood clots which in the case of a person at risk of a stroke or heart attack, may be dangerous.

VITAMIN D and DIGOXIN - Increases in blood levels of calcium caused by taking vitamin D may increase the effect of digoxin and result in cardiac arrhythmias.7

IRON and LEVODOPA (as above) - Iron is a supplement taken to enhance the amount of iron in the blood. If levodopa is taken at the same time the effectiveness of the drug is lowered. It should be noted that Iron combines with several drugs in the gastrointestinal tract with varying results.

IRON and ZINC - antibacterials - Iron and zinc combine with several antibacterial drugs to form insoluble complexes. This lowers the effectiveness of the drugs. Some examples of the drugs include the tetracyclines, and some of the 4-quinolones (e.g. Noroxin) and penicillamine.

IRON and ANTACIDS - Iron should not be taken as a supplement when taking any antacid as the pH in the intestine is increased by antacids and the iron is not adequately absorbed. This is a possible difficulty for pregnant women who are often taking both products and should be told to separate the doses of iron and antacid by two hours.

CALCIUM and TETRACYCLINES - Calcium taken as a supplement lowers the absorption of tetracyclines.
Calcium supplements should not be taken with any thiazide diuretics as the excretion of the calcium is reduced by the drugs, which can result in too much calcium being stored in the body.

ANTI-VITAMIN DRUGS - Certain drugs such as isoniazid, monoamine oxidase inhibitors, methotrexate, phenytoin (Dilantin), primidone (Mysoline), pyrimethamine (e.g. Daraprim), sulfasalazine and trimethoprim have anti-vitamin effects and as a result are considered vitamin antagonists.

These effects include increasing the body's requirement for a certain nutrient, preventing nutrients from being absorbed and preventing the proper use of nutrients in the body. Examples are as follows:

FOLIC ACID - PREGNANCY - Pregnant women or women contemplating pregnancy who are taking anticonvulsive drugs should take folic acid as a supplement. Certain anticonvulsive drugs can deplete the body of folic acid, which, recent research has confirmed, may cause an infant to be born with a neural tube defect. Dosing with folic acid may require an increase in anti-convulsive medication as folic acid acts as an antagonist against these drugs.8

PHENYTOIN (DILANTIN) - Phenytoin (Dilantin) and other anticonvulsants may be antagonistic to vitamin D metabolism in the body causing a deficiency of the vitamin. It has been suggested that children on this type of therapy should take vitamin D as a supplement. Vitamin D is important for normal growth and development.

ORAL CONTRACEPTION - Studies suggest that oral contraceptives interfere with the metabolism of several vitamins including vitamin A, vitamin C, vitamin B12 and folic acid. More recent formulations of oral contraceptives have a reduced amount of oestrogen and the need for supplements may, therefore, be reduced.

POTASSIUM - Potassium (including salt substitutes) should not be taken with any ACE inhibitors such as Capoten. Any potassium sparing diuretic should not be taken with a potassium supplement because of the possible harmful effects of increased potassium in the body.

  • Management

    If you are taking medication, check with your Doctor to see if there are any possible nutrient interactions that occur as a result of taking the drug. Ask your Doctor if any vitamin or mineral supplements may be contraindicated (not recommended) with the medication you are taking.

  • Notes

    References

    1. An FDA Guide to Dietary Supplements. Food and Drug Administration. Last updated January 1999. [cited 5 December 2000]. Available at URL: http://vm.cfsan.fda.gov.
    2. An FDA Guide to Dietary Supplements. Food and Drug Administration. Last updated January 1999. [cited 5 December 2000]. Available at URL: http://vm.cfsan.fda.gov.
    3. Vitamins: of surface value?. Australian Pharmacist 1998 17(5):302-305.
    4. Vitamins: of surface value?. Australian Pharmacist 1998 17(5):302-305.
    5. Vitamins: of surface value?. Australian Pharmacist 1998 17(5):302-305.
    6. Vitamins: of surface value?. Australian Pharmacist 1998 17(5):302-305.
    7. Vitamins: of surface value?. Australian Pharmacist 1998 17(5):302-305.
    8. Vitamins: of surface value?. Australian Pharmacist 1998 17(5):302-305.


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