Drug interactions are changes in the effect of one drug due to the effect of either another drug taken at the same time (drug-drug interactions) or food consumed while the drug is being taken (drug-food interactions).
Some drugs are deliberately combined for administration because there are beneficial effects to be derived. Generally, drug interactions are unwanted and harmful. They may either serve to intensify or diminish the desired effect of a particular drug, or worsen its side effects. Although most drug-drug interactions involve prescription drugs, they can occur with nonprescription or over-the-counter (OTC) medications—the most common of these being aspirin, antacids, and decongestants.
Individuals under the care of more than one health care practitioner are at highest risk for drug interactions because each practitioner may not be aware of drugs being prescribed by the other. The patient needs to make each practitioner fully aware of all medications being taken. This includes prescription and nonprescription medications, and herbal remedies. A practitioner may not think to ask a patient about OCT medications or herbal supplements, and the patient may not think to inform their practitioner they are taking these substances because they do not think of OTCs or herbals as medications. The more drugs being consumed, the greater the risk of developing a drug interaction. This risk also increases with the amount of drug taken and the tendency of particular drugs to interact adversely with each other. The incidence of adverse drug reactions increases with the age of the patient; the risk is three times greater for older people than for younger adults.
When the effect of one drug in the body is altered by the presence of another, one drug may increase or decrease the effects of the other with harmful results. Two drugs taken together may produce a new and dangerous reaction, or they may be in opposition to each other in their actions. Two similar drugs taken together may produce an effect that is greater than would be expected from one drug. This effect is called potentiation. One drug may also affect the rate at which the kidneys excrete another drug by altering the acidity of the urine. This, in turn, affects the excretion of other drugs. Vitamin C in large doses can do this. Although not considered a drug by some, alcohol is a drug that does affect bodily processes and is often responsible for drug interactions. OCT drugs can interact with each other as well as with prescription drugs. For example, many cough medicines contain alcohol which, if taken with antihistamine medications, could increase sleepiness and decrease alertness.
Examples of drug interactions include:
- salt substitutes interacting with potassium-sparing diuretics (agents that promote urine secretion) to increase blood potassium levels and cause nausea, vomiting, diarrhea, muscle weakness, and possibly cardiac arrest
|Interactions between drugs and commonly used modalities|
|Modality||Use||Drugs with complementary/synergistic effects||Drugs with antagonistic effects|
|SOURCE: Ciccone, C.D. Pharmacology in Rehabilitation. 2nd ed. Philadelphia: F.A. Davis Co., 1996.|
|Cryotherapy: cold/ice packs, ice massage, cold baths, vapocoolant sprays||Decreased pain, edema, and inflamation||Anti-inflammatory steroids (glucocorticoids); nonsteroidal anti-inflamatory analgesics (aspirin and similar NSAIDs)||Peripheal vasodilators may exacerbate acute local edema.|
|Muscle relaxation and decreased spacticity||Skeletal muscle relaxants||Nonselective cholinergic agonists may stimulate the neuromuscular junction.|
|Superficial and deep heat (local application): hot packs, parafin, infrared, fluidotherapy, diathermy, ultrasound||Decreased muscle/joint pain and stiffness||NSAIDs; opioid analgesics; local anesthetics||—|
|Decreased muscle spasms||Skeletal muscle relaxants||Nonselective cholinergic agonists may stimulate the neuromuscular junction.|
|Increased blood flow to improve tissue healing||Peripheral vasodilators||Systemic vasocontrictors (e.g., α-1 agonists) may decrease perfusion of peripheral tissues.|
|Systemic heat: large whirlpool, Hubbard tank||Decreased muscle/joint stiffness in large areas of the body||Opioid and nonpioid analgesics; skeletal muscle relaxants||Nonselective cholinergic agonists may stimulate the neuromuscular junction.|
|Ultraviolet radiation||Increased wound healing||Various systemic and topical antibiotics||—|
|Management of skin disorders (acne, rashes)||Systemic and topical antibiotics and anti– inflammatory steroids (glucocortroids)||Many drugs may cause hypersensitivity reactions that result in skin rashes, itching.|
|Transcutaneous electrical nerve stimulation (TENS)||Decreased pain||Opioid and nonopioid analgesics||Opioid antagonists (naloxone)|
|Functional neuromuscular electrical stimulation||Increased skeletal muscle strength and endurance||—||Skeletal muscle relaxants.|
|Decreased spasticity and muscle||Skeletal muscle relaxants spasms||Nonselective cholinergic agonists may stimulate the neuromuscular junction.|
- decongestants interacting with diuretics to increase blood pressure
- antacids interacting with anticoagulants (blood thinning drugs) to slow down absorption of the prescribed drug or interacting with absorption of other drugs—such as the antibiotic tetracycline—and thus prolonging an infection
- aspirin increasing the effect of blood thinning drugs
- antihistamines increasing the sedative effects of barbiturates (sleeping pills), tranquilizers, and some pain relievers
- iron supplements binding with antibiotics in the stomach, preventing absorption of the antibiotic into the bloodstream
- antihypertensive medications mixed with digitalis (Lanoxin) resulting in abnormal heart rhythms
- anticoagulants mixed with sleeping pills resulting in decreased effectiveness of the anticoagulant
- antibiotics taken by women on the low-dose birth control pill causing decreased effectiveness of the pill
- nonsteroidal antiinflammatory drugs (NSAIDs) causing the body to retain salt and fluid that can oppose or antagonize the effectiveness of a diuretic
- beta-blockers, such as propanolol, counteracting certain drugs taken for asthma
Certain drug-food combinations can produce dangerous side effects as well. Food can speed up or slow down the action of a medication, and some drugs may prevent the absorption of vitamins and minerals from food. Drugs may also alter the way the body uses nutrients, as well as altering the taste sensation. Chemicals in cigarette smoke can increase the enzyme activity of some liver enzymes, which, in turn, reduces the effectiveness of some pain relievers and some drugs used for lung conditions. The antibiotic tetracycline is not absorbed appropriately if it is taken within an hour of drinking milk or eating other dairy products or foods containing calcium.
To help avoid drug interactions, patients should inform their health care practitioners of any medical problems they have or have had, maintain a list of drugs—both prescription and nonprescription—taken during the few weeks prior to their visit to their practitioner, and give the list to their practitioner. They should also advise their practitioner of any allergies or unusual reactions to drugs, food, or other substances, and make
Although drug interactions can create serious health risks, they can be avoided with appropriate education (of the patient) and history-taking (by the health care practitioner). The health care practitioner needs to take the time to ask pertinent questions about an individual's history, including diet, nutritional intake, and medication regimen which should include nonprescription drugs and herbal remedies. Although it is often difficult for the practitioner to spend an appropriate amount of time with patients in this era of health care management, it could ultimately save time and money and prevent serious consequences to the health of the patient. Healthcare providers need to stay informed and up-to-date regarding drug-drug and drug-food interactions.
Healthcare professionals are capable of exerting a measurable impact on the lives of individuals who are taking drugs for either medical or nonmedical reasons. Education is the foundation for patient care with regards to potential drug interactions. It is the responsibility of the care provider to take the time to inform patients of potential interactions with drugs and food, as well as to take a complete patient history. A patient's family should be included in the educational process so they may recognizing possible interactions by changes in behavioral patterns or demeanor.
Potentiation—Making effective or active, or more effective or active; synergistically augmenting the activity of one drug with another.
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Linda K. Bennington, C.N.S.