Drug-Supplement Interactions: Patient Awareness Is Key

Pharmacy TimesOctober 2013 Diabetes
Volume 79
Issue 10

As supplement use escalates, patients must be made aware of potential drug"supplement interactions.

As supplement use escalates, patients must be made aware of potential drug—supplement interactions.

Results from a 2012 survey report show that an estimated 63% of adults in the United States take 1 or more nutritional/dietary supplements, indicating a 3% increase from the previous year.1 These supplements are often used in conjunction with prescription medications. While many health care professionals are aware of the significance of screening for drug—drug interactions, it is also critical to address potential drug–supplement interactions.

Some experts believe that drug—supplement interactions are not as easily recognized or managed and therefore necessitate a better understanding and more clinical interventions to improve overall patient care and safety.2,3 Results from a recent study reported that more than 50% of patients with chronic diseases or cancer may use nutritional supplements and because these patient populations are likely to take multiple medications, there is an increased risk for drug—supplement interactions.3 A drug—supplement interaction is considered to be significant if it changes a pharmacotherapeutic response or compromises a patient’s nutritional status.4

As the popularity of supplement use continues to escalate, health care professionals should ensure that patients are aware of the potential for drug—nutrient interactions. According to the Handbook of Drug—Nutrient Interactions, in some cases, potential drug—supplement interactions can be of clinical importance, but often go unrecognized by many consumers and even health care professionals.4 As a result, a drug—supplement interaction may be a contributing factor in ineffective therapy, adverse drug effects, and increased risk of deficiencies or toxicities and can negatively affect a patient’s nutritional status.4 The need to augment awareness about drug—supplement interactions is critical to patient safety, preventing potential interactions, and ensuring optimal therapeutic effects because some studies report that nearly 70% of patients taking prescription drugs do not inform their primary health care provider about their use of nutritional supplements.5 Health care professionals should encourage patients to maintain a comprehensive list of all medications, including supplements.

Many patients perceive nutritional supplements as being safe and natural without any adverse effects, and the consequences of potential drug—supplement interactions are not always widely understood or known.6 While drug–supplement interactions can be harmful to anyone, the elderly patient population may be considered at greatest risk because this patient population is more likely to take multiple medications, including nutritional–dietary supplements (Online Table 1).7-9

Table 1: Individuals at Increased Risk of Drug—Nutrient Interactions4,8-10

  • Patients with chronic diseases, especially those taking multiple medications
  • Elderly individuals
  • The pediatric population
  • Patients with poor or compromised nutritional status
  • Patients with poor overall health status
  • Pregnant women

Classifying Drug—Supplement Interactions

Interactions between drugs and nutritional supplements can be classified into 2 general groups—pharmacokinetic and pharmacodynamic interactions (Online Table 2).4,15-18

Table 2: Classification of Drug—Supplement Interactions 4,15-17

Pharmacokinetic interactions take place when an alteration in the absorption, distribution, metabolism, or elimination of the drug occurs. The action may increase or diminish pharmacologic activity. Examples of these types of interactions include those that involve the cytochrome P450 enzyme system.

Pharmacodynamic interactions transpire between drugs and supplements when there is any alteration in the action of a medication. These forms of interactions occur when the activity of 1 drug augments or antagonizes the other agent. Pharmacodynamic interactions are possible when the supplement’s pharmacology is comparable to or the opposite of the drug that the patient is taking.

Drug—Supplement Interactions

There are a host of potential drug—supplement interactions, and many clinical studies have investigated their significance and effects (Online Table 3). One study reported that there are several herbal and dietary supplements that can cause drug–supplement interactions, especially among patient populations that are being treated for central nervous system (CNS) or cardiovascular disorders.3,11 Results from this study also found that some classes of pharmacologic agents with the greatest potential for drug—supplement interactions include antithrombotic medications, cardiovascular medications, sedatives, antidiabetic agents, and antidepressants.3,11 In addition, results from a recent study found that agents such as warfarin, insulin, aspirin, digoxin, and ticlopidine had the most interactions reported with herbal and dietary supplements.3,10,11 The study also concluded that supplements containing St. John’s wort, calcium, magnesium, iron, and ginkgo had the greatest number of documented interactions with prescription medications.3,11 Researchers concluded that by enhancing communication between patients and clinicians regarding the use of prescription drugs and supplements, many of these potential interactions could be decreased or even prevented.3,11,12

Table 3: Examples of Potential Supplement—Drug Interactions9,15,18

Dietary/Herbal Supplement



Precautionary Measures

Vitamin A or vitamin E

(high doses)


Increases anticoagulation effects

Take only recommended daily required intakes

Vitamin B6


Levodopa antagonism, decreased effectiveness

Avoid B6 supplementation or, if possible, recommend levodopa/carbidopa combination

Phenytoin and phenobarbital

Decreased serum level of phenytoin and phenobarbital

Monitor patient levels routinely

Vitamin B12

Metformin, anticonvulsants, ascorbic acid supplements, antacids, antibiotics

Decreased absorption of cyanocobalamin

Clinical significance is unknown

Vitamin D

Phenytoin , carbamazepine,

or barbiturates

Increase metabolism of

vitamin D

Ensure adequate daily intake of vitamin D

Vitamin K

Long-term therapy with

broad-spectrum antibiotics

Vitamin K deficiency induced by decreased gut flora

Ensure adequate Vitamin K intake


Decreased anticoagulation, increased risk of thromboembolism

Keep daily intake of vitamin K consistent

Folic acid


May inhibit folic acid absorption, leading to megaloblastic anemia

(note: Folic acid supplementation may de-

crease the serum phenytoin level and interfere with seizure control due to subtherapeutic levels of phenytoin)

Monitor phenytoin level routinely and monitor for megaloblastic anemia; consult with neurologist regarding supplementation


Oral hypoglycemics

Decreased hypoglycemic effects

Monitor blood glucose regularly


Tetracyclines and fluoroquinolones

Decreased antibiotic and iron absorption

Take antibiotics at least 2 hours before or 6 hours after taking iron



Inhibits Ca2+ absorption from gut, thus increasing risks for bone fractures and osteoporosis

Recommend Ca2+ supplementation.


Tetracyclines, fluoroquinolones, and levothyroxine

Decreased antibiotic


Separate dosing by 2 hours before or 6 hours after antibiotic

Garlic Supplements

  • Warfarin
  • Saquinavir
  • Oral Contraceptives

1) Increased anticoagulation

2) Decreased drug level

3) Decreased effectiveness

Evidence unknown;

consult primary health care provider before use

St. John’s wort

1) CYP3A4 substrates ( ie, simvastatin, alprazolam, tacrolimus, and warfarin)

2) Digoxin

1) Reduced drug level

2) Decreased drug level

Consult primary health care provider and avoid combining with known drug interactions


1) Antidiabetic agents

2) Monoamine oxidase inhibitors

1) Possible additive effect of hypoglycemia

2) Interaction possible based on reports of manic-like symptoms

1) Consult primary health care provider and maintain routine blood glucose

2) Avoid combination of the 2 agents



Possible additive effect

Use with caution or avoid combination


MAOIs, SSRIs, and tricyclic antidepressants

Increases melatonin level

Consult primary health care provider

Coenzyme Q 10

Certain chemo drugs

May increase or decrease toxicity of certain chemo


Consult primary health care provider

MAOIs = monoamine oxidase inhibitors; SSRIs = selective serotonin reuptake inhibitors.

Adapted from references 9, 15-18.

Studies have also demonstrated that even though there are several possible drug—supplement interactions, only a moderate number of them have significant interactions, and antithrombotic agents were most likely to interact with supplements and have many documented interactions.6,10,13,14 For example, patients on anticoagulant therapy were at greater risk of increased anticoagulant activity when the therapy was in combination with supplements that have antiplatelet properties, such as fish oil, garlic, ginseng, ginkgo, or flaxseed oil.15,16 Studies have also reported that St. John’s wort decreased therapeutic levels of agents such as verapamil, statins, antiretrovirals, and digoxin.15 St. John’s wort has been associated with serotonin syndrome in patients taking prescription psychotropic agents.16 In addition, when used in combination, St. John’s wort has been reported to induce the metabolism of drugs such as indinavir, oral contraceptives, and cyclosporine.16

All potential nutrient—drug interactions are too numerous to mention, but examples include the use of calcium, magnesium, and zinc, which may decrease the absorption of antibiotics such as tetracyclines and fluoroquinolines.8,9 Metformin and antiulcer agents may diminish absorption of vitamin B12 and folic acid, while individuals taking antidepressants may have lower levels of vitamins B6, B12, and D.8,9 The use of garlic supplements has been known to interact with anticoagulants and pose an increased risk of excessive bleeding.8,16 Individuals taking CNS drugs are at greater risk for interactions as well.8,9 Supplements such as ginkgo and ginseng may interact with these types of drugs. In addition, agents such as antiseizure drugs often adversely affect certain B vitamin levels, and several are classified as antifolates.8,9,16

Prior to recommending any nutritional supplements or a new medication, health care professionals must evaluate a patient’s medical history and medication profile to screen for potential drug—supplement interactions or contraindications. It is also crucial that patients be made aware of these potential interactions as well as understand the importance of notifying their primary health care provider of all supplements that they are taking, including alternative and complementary products. In March 2012, the National Institutes of Health (NIH) National Center for Complementary and Alternative Medicine held a meeting to discuss the issue of dietary supplement–drug interactions. A summary of the meeting can be found at http://nccam.nih.gov/news/events/druginteraction. The NIH Office of Dietary Supplements and the NIH National Center for Complementary and Alternative Medicine websites also provide excellent resources on drug—supplement interactions. The NIH Drug–Nutrient Interaction Task Force also provides excellent patient resources on its website at www.cc.nih.gov/ccc/patient_education/important_drug_food_info.html.

Ms. Terrie is a clinical pharmacy writer based in Haymarket, Virginia.


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