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Vitamin Interactions With Drugs

By: Suzanne Elvidge BSc (hons), MSc - Updated: 15 Sep 2012 |
Vitamin Interactions With Drugs

Vitamins are nutrients that are essential to daily life. They are available as supplements from supermarkets, health food shops and chemists, and it’s easy to think of them as completely harmless.

Like foods, some vitamins can interact with drugs, changing how they are absorbed or how they act. Some can improve the effectiveness of drugs, or reduce the side effects, and others can make the drugs less effective, or make side effects worse. If in any doubt about taking vitamins and drugs together, talk to a pharmacist or doctor.

Vitamin A (retinol)

High doses of vitamin A taken with tetracycline antibiotics may cause intracranial hypertension, an increase in the pressure of the fluid around the brain, and may increase the risk of bleeding if taken with blood-thinning drugs (anticoagulants).
The combination of vitamin A and antacids may help ulcers to heal.

Vitamin B Complex

The B vitamins can reduce the absorption and the effects of the tetracycline antibiotics.
Vitamin B2 (riboflavin) and vitamin B6 can make tricyclic antidepressants more effective.

Vitamin B3 (niacin)

Vitamin B3 may increase the risk of bleeding if taken with blood-thinning drugs. It may also increase the effect of antihypertensives (taken to reduce blood pressure).

Vitamin B3 binds to bile-acid sequestrants (drugs used to lower cholesterol) so may make them less effective.

The combination of vitamin B3 and simvastatin (a cholesterol-lowering drug) may slow the onset of heart disease, but may also increase the risk of side effects.

Vitamin B6 (pyridoxine)

Vitamin B6 can make phenytoin, used in epilepsy, less effective.

Vitamin B6 can reduce the side effects of the anticancer drugs 5-fluorouracil and doxorubicin, without changing their effectiveness. However, while vitamin B6 can reduce the side effects of levodopa, a drug taken for Parkinson’s disease, it can also reduce how effective it is. Only take vitamin B6 to reduce side effects at the advice of a doctor.

Vitamin B9 (folic acid)

Vitamin B9 may reduce levels of phenytoin and other anticonvulsants, reducing its effectiveness.

Vitamin C

High doses of vitamin C can increase the amount of aspirin, paracetamol and non-steroidal anti-inflammatory drugs (NSAIDs – painkillers like ibuprofen) that is in the body by reducing the amount that is lost in the urine – this could make it more effective, or could increase side effects.

Some antacids contain aluminium, and vitamin C can increase the amount of aluminium absorbed, which could make side effects worse.

Vitamin C may help the body not to become tolerant to nitrate drugs (used in heart failure), which could keep them effective for longer.

Vitamin C may increase levels of tetracycline antibiotics, making them more effective but potentially increasing the incidence of side effects.

Vitamin D

Vitamin D may make doxorubicin (used to treat cancer) more effective.

Vitamin E

Vitamin E may reduce the absorption of tricyclic antidepressants, beta-blockers (antihypertensives, used to treat high blood pressure) and chlorpromazine (a phenothiazine antipsychotic, used in schizophrenia). Taking vitamin A, C and E with mebendazole (an antiparasitic used to treat intestinal worms) may reduce its effectiveness. Combining vitamin E with blood thinning medication may increase the risk of bleeding.

Vitamin E may reduce the side effects of AZT, used to treat HIV/AIDS.

Vitamin K

Vitamin K reduces the effects of warfarin, used in the prevention of blood clots (blood thinning).

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