Drug interactions

From the occasional glass of wine to powerful medications used to fight cancer, drug interactions have the ability to affect nutritional status and health in a variety of ways. There are more than 100,000 medications available in America and almost two billion prescription drugs are dispensed every year.

Many of these drugs affect the way the body uses vitamins and minerals from food. In most cases, medications deplete nutrients gradually and deficiencies may not be noticed until stores are exhausted. Alcohol also affects nutrient status and while light drinking may have some beneficial effects, heavy use carries a high risk of malnutrition. It is important to understand how drugs affect the body in order to minimize adverse effects, enhance recovery and maintain optimal health.

Drug interactionsDrug-induced nutrient deficiencies

In most cases, unless drugs are used on a long-term basis, the effects on individual nutrients are not long lasting. Vitamin and mineral deficiencies are more likely to occur in people who take medications for long periods, particularly if their diets are usually low in essential nutrients. This type of medication use typically occurs in those with gastrointestinal disorders, diabetes, cancer, epilepsy, cardiovascular disease and autoimmune diseases. Alcohol, tobacco and oral contraceptives also fall into this category.

Elderly people tend to take more prescription and non prescription medications than any other group and usually also have increased needs, reduced appetite and absorption. This combination means that they are at high risk of drug interactions. Children, pregnant women, those on reduced calorie diets and alcoholics are also susceptible.

Prescription and nonprescription drugs can interact with nutrients in a number of ways.

Drug interactions: Alterations in appetite

Many medications can increase or decrease appetite, which can lead to a change in the amounts and types of food a person eats. Some drugs, such as some antihistamines and antidepressants can increase appetite. In some cases this may be desirable, but in others there may be an increased risk of obesity and the complications this causes. Many medications reduce appetite. Some are intended to do so, such as amphetamines for weight loss, and others do so as a side effect. If weight loss occurs in those who are already underweight, malnutrition may occur.

Some drugs affect the gastrointestinal system, causing nausea, vomiting, constipation or diarrhea. Others alter the production of saliva and the ability to taste, reducing a person's ability to enjoy food. Some drugs affect mood and alter the desire to eat. Drugs with these effects contribute to reduced intake and can lead to vitamin and mineral deficiencies.

Drug interactions: Alterations in absorption

Some medications alter the absorption of vitamins and minerals so that even if dietary intake is sufficient, the body may not absorb enough for optimum health. Other medications bind vitamins and minerals in the intestine and reduce the ability of the body to absorb them; for example mineral oil laxatives reduce absorption of the fat soluble vitamins A, D, E and K.

Other drugs reduce the amount of time food spends in the intestine which means that the body may not have time to absorb all it needs. Others may block absorption sites or alter them so vitamins and minerals cannot pass through. Some drugs may affect absorption by changing the acidity of the stomach and intestines. In some cases the production of digestive juices essential for breakdown and absorption may be reduced.

Drug interactions: Alterations in metabolism

Certain drugs alter the availability, storage and use of nutrients. Some of these chemically resemble vitamins and minerals and can bind to the same active sites on an enzyme. However, these drugs may not have the same beneficial effects as a vitamin and this can lead to deficiency symptoms. Other drugs may alter storage.

Some medications increase excretion of certain nutrients. This means that a particular vitamin or mineral is not retained in the body long enough to have beneficial effects; for example, some diuretic drugs increase potassium excretion.

Anti-inflammatory medications

The antigout drug, colchicine, reduces the absorption of vitamin B12, increases tissue loss of calcium and potassium and reduces the absorption of folic acid, iron and vitamin A. Nonsteroidal anti-inflammatory drugs (NSAIDS) may cause kidney damage, leading to high potassium levels.

Sulfasalazine can inhibit the absorption and lower the plasma concentrations of folic acid. Thiamin and riboflavin requirements may also increase. Indomethacin may impair vitamin C and thiamin metabolism.

Beta blockers

Beta blockers, which are used to treat high blood pressure and some other types of cardiovascular disease, have been shown to interfere with the production and function of coenzyme Q10, and to adversely affect heart function. This may explain why, in some cases, long-term therapy with beta blockers can lead to congestive heart failure. Coenzyme Q10 therapy in combination with beta blockers may be beneficial.


Corticosteroid drugs are widely used in medical treatment for autoimmune diseases such as arthritis, chronic obstructive lung disease, asthma and allergic conditions. Treatment with high doses of corticosteroids causes osteoporosis, particularly in the type of bone found in the lumbar spine.

Corticosteroid cause osteoporosis by several mechanisms; they decrease levels of sex hormones and also directly affect the cells which build and break down bone. They also decrease the absorption of calcium from the intestine. Corticosteroids may also raise folic acid, vitamin B6 and vitamin C requirements.

Vitamin D and calcium supplements may be useful in preventing bone loss in patients taking corticosteroids. In a 1996 study, researchers showed that calcium and vitamin D supplements can help prevent this loss. In the two-year study, 96 patients with rheumatoid arthritis, 65 of whom were taking corticosteroid drugs, were given 1000 mg calcium and 500 IU vitamin D per day or placebo. The researchers analyzed the bone mineral density of the lumbar spine and femur for one a year. In those patients taking corticosteroid drugs and placebo, losses of bone mineral density were seen. In those taking the supplements, gains were seen; and in those not taking corticosteroids, the supplements did not appear to affect bone mineral density.

Arthritis medications

D penicillamine is used in severe and disabling rheumatoid arthritis. It may affect nutrient intake by causing loss of appetite and gastrointestinal disturbances. It can also reduce absorption of iron and zinc and should not be taken with foods containing these minerals. Experts suggest that at least a two-hour period should elapse between the administration of iron salts and oral penicillamine doses. Vitamin B6 needs may also be increased with long-term use of D penicillamine. Supplements in doses of 25 mg per day may be useful.


Aspirin is used as a painkilling and temperature-reducing drug. Frequent use of aspirin may cause bleeding in the stomach and intestines. This may lead to iron deficiency if aspirin is taken for long periods. Patients with vitamin K deficiency

tihould use aspirin cautiously as increased bleeding may result. Vitamin Amay protect against stomach ulcers and bleeding in those taking aspirin.

Aspirin may also lead to deficiencies of folic acid, thiamin, vitamin C, potassium and vitamin B12 if used for a long time. Calcium decreases the absorption of aspirin if taken at the same time. The nicotinamide form of niacin cuuses flushing when taken in large doses. This is caused by blood vessel dilatation, and pretreatment with aspirin may inhibit this effect, which some people find unpleasant.

Asthma medications

Long-term therapy with theophylline, a drug often given to asthmatic patients, lowers vitamin B6 levels. Vitamin B6 supplements may be useful in preventing the side effects of the drug which include headaches, nausea, sleep disorders and convulsions.


Caffeine may increase calcium and potassium losses and reduce zinc absorption. This may contribute to disorders such as high blood pressure and osteoporosis. Caffeine may also raise thiamin requirements. Most studies suggest that the harmful effects of caffeine occur in those who drink more than two cups of coffee per day.

Cardiovascular disease medications

The effect of the heart stimulant drug, digoxin, can be toxic when magnesium and potassium concentrations are too low and when calcium is too high. However, digoxin may not be effective in patients with low calcium levels and small amounts of supplements may be necessary. It is recommended that serum potassium and calcium be monitored regularly in patients receiving digoxin. Digoxin may also lead to thiamin deficiency.

Cholesterol-lowering drugs

In recent years, the"statin"drugs, lovastatin, pravastatin, and simvastatin have become widely used to treat high blood cholesterol. These medications work by inhibiting an enzyme known as HMG-CoA reductase, and they are very effective

in lowering cholesterol levels. However, this enzyme is also responsible for production of coenzyme Q10. Because of this, the cholesterol-lowering effect of these drugs is accompanied by an equivalent lowering of coenzyme Q10 levels. In patients with existing heart failure, lovastatin has been shown to cause increased heart disease with life-threatening results in some patients. Coenzyme Q10 supplements may help to prevent some of the adverse effects of these widely used drugs.

The cholesterol-lowering drug, cholestyramine, reduces the absorption of fat soluble vitamins A, D, E and K, and may lead to deficiencies. Water soluble supplements of these vitamins may be useful. Folic acid, vitamin B12 and iron levels may also be adversely affected, leading to anemia. Cholestyramine may also increase calcium excretion. Colestipol may also lead to folic acid and fat soluble vitamin deficiencies. Niacin may increase the effectiveness of colestipol.

Diabetes medications

The antidiabetic drug, metformin, can inhibit the absorption of vitamin B12, leading to anemia. Other B vitamin deficiencies are also common in patients taking this drug. Niacin may affect the control of blood sugar in diabetics and should be used with caution in patients taking metformin.

Estrogen therapy

Estrogen-containing drugs include oral contraceptives and the hormones used in hormone replacement therapy (HRT). They are often used for long periods and can affect the absorption and use of several nutrients. Some of these effects are beneficial and others may be harmful.


It has been suggested that estrogen can increase the risk of certain cancers, including those of the breast and cervix. These links have not been confirmed but some studies suggest that estrogen may increase free radical formation. If this is the case, antioxidant vitamin supplements may be useful in protecting against oxidative damage. Oral contraceptive use may also decrease vitamin C and beta carotene levels. Large doses of vitamin C may cause higher blood levels of estrogen when taken at the same time as the contraceptive pill. It is best to take vitamin C supplements separately.


HRT is used to increase bone mineral metabolism in postmenopausal women who are at risk of osteoporosis. Estrogen increases calcium absorption, decreases calcium excretion and increases vitamin D levels, thus improving the deposition of calcium into bone.


Some research suggests that there is a higher risk of abnormalities in cervical tissue in women using oral contraceptives. Supplements are beneficial in preventing cervical dysplasia in these women.

Vitamin B6

As estrogen may affect vitamin B6 metabolism and increase needs, supplements may be beneficial for pregnant women, those on the contraceptive pill or those on hormone replacement therapy (HRT) who suffer from mood swings and depression. Vitamin B6 is sometimes known as the women's vitamin.

Other nutrients

Blood levels of vitamin B12, thiamin, riboflavin, magnesium and zinc may be lowered by the contraceptive pill. Copper and vitamin A levels may be raised. The contraceptive pill may reduce menstrual bleeding which decreases iron loss.

High blood pressure medications

There are various types of high blood pressure medications and these vary in their effects on vitamins and minerals. ACE inhibitors such as enalapril raise potassium levels. They should not be used with potassium supplements as heart problems can occur with excessive levels. People taking potassium sparing diuretics such as spironolactone should also avoid potassium supplements as body levels may become too high. Spironolactone may also reduce the availability of vitamin A. Loop diuretics such as bumetanide may cause excessive potassium loss and supplements may be necessary.

Low potassium levels is one of the most common adverse effects associated with thiazide diuretic therapy and can lead to cardiac arrhythmias. This effect is especially important to consider in patients receiving cardiac glycoside therapy (such as digoxin) because potassium depletion increases the risk of toxicity of

this drug. Potassium supplements may be necessary in such cases, although they should be used cautiously in patients receiving digoxin. Thiazide diuretics may also decrease calcium excretion and increase magnesium excretion. Magnesium supplements may be useful in patients on long-term diuretic therapy. Zinc absorption may also be reduced. Hydralazine may increase the demand for vitamin B6 as it enhances excretion. Long-term use of frusemide may lead to thiamin deficiency.


Long-term use or overdosage can cause abdominal pain, nausea/vomiting, loss of weight, muscle weakness, laxative dependence, and low potassium and calcium levels. Mineral oil laxatives bind fat soluble vitamins and reduce absorption.

Osteoporosis medications

Taking oral etidronate with vitamin and mineral supplements that contain calcium salts, iron salts such as ferrous sulfate or magnesium salts may be inadvisable. These salts can interfere with the absorption of etidronate, and should not be taken within two hours of the drug. Even though calcium salts should not be taken at the same time as etidronate, patients need to maintain an adequate intake of calcium and vitamin D to avoid low calcium levels.

Psoriasis medications

The vitamin D-derivative drug, calcipotriene, should not be used with high doses of vitamin D and calcium.

Thyroid medications

Excess intake of iodine/iodide can decrease the efficacy of propylthiouracil.

Sedatives and tranquilizers

Barbiturate drugs may enhance excretion and metabolism of vitamin C, and reduce the conversion of vitamin D to its active form.

Vitamin A-derivative drugs

Vitamin A supplements should not be used with the vitamin A-derivative drugs tretinoin, isotretinoin and etretinate. These drugs are used to treat skin disorders such as acne and can be toxic if taken with high levels of dietary vitamin A.

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