By Ron Conte, Pharm.D.
Drug interactions are numerous. One drug may interact with another drug or several drugs, also termed a drug-drug interaction. Some drugs will interact with foods (drug-food interaction) while other drugs may interact with a patient’s disease state, or vice versa. Drugs may interact with over-the- counter (OTC, non-prescription) drugs or with herbal products while other drugs will interfere with specific laboratory tests.
There are two major classifications of drug interactions: potentiation and antagonism. Potentiation is also termed synergistic or additive since the end-result is to enhance some common effect of both agents. For example, morphine and lorazepam (generic Ativan) may be used in combination to produce more sedation and/or decrease the pain level. As for antagonistic drug interactions, one drug’s effect may be blocked by another drug, food, or even a disease state. The small group of agents known as antidotes to specific drugs are included as drug-drug antagonistic interactions. When vitamin K is used to treat excessive bleeding due to warfarin (generic Coumadin, an anticoagulant), it is an example of an antidote counteracting the excessive effects of an anticoagulant.
I am sure you have been told to not take certain medication with food. Why? The absorption of certain medication may be blocked by specific ingredients in foods. For example, levofloxacin (an antibiotic, also known as Levaquin) should not be taken with dairy products, in particular, calcium. The calcium binds levofloxacin in the intestine which limits levofloxacin’s absorption and thereby lessens its effectiveness as an antibiotic.
There are many types of drug-food interactions. The example in the above paragraph of warfarin and vitamin K could also be classified as a drug-food interaction since vitamin K is found in green, leafy vegetables.
A class of drugs known as beta-blockers, e.g., propranolol (generic Inderal) and others, may worsen an asthmatic patient’s condition. This is an example of a drug-disease state interaction. Rx medication for anxiety or sleep such as diazepam (generic Valium) and an OTC for allergic symptoms, diphenhydramine (generic Benadryl) may cause excessive drowsiness when taken together. This is an example of a (Rx) drug and OTC med interaction.
A good example of a drug-herbal product interaction is with the intake of St John’s Wort and diazepam. St John’s Wort may lessen the effect of diazepam. Prednisone, a steroid used to treat inflammatory conditions, may increase the blood sugar level. So, if a diabetic patient is taking prednisone and gets a random blood sugar test, there may be an unusually higher level of sugar detected in the blood. This is an example of the drug-laboratory test interference.
The more Rx drugs, OTC meds, and herbal products you take, the greater the risk of developing a clinically-significant drug interaction. The literature states that there is a 100% chance of experiencing a drug interaction when five or more drugs are taken. There are many drug interaction software programs available for use by healthcare professionals and there are hundreds of possible drug interactions. Not all are clinically-significant.
What is meant by “clinically-significant”? Some drug interactions only alter body systems (e.g., heart, brain, lung, etc.) less than 1% of the time, or the end-result does not noticeably affect body systems. These are termed clinically insignificant drug interactions. The pharmacist is more highly trained than any other healthcare professional in determining clinically significant drug interactions. Check with your pharmacist to find out if you will potentially face, or have experienced, a clinically significant drug interaction.
If you have any questions, or additional interest in this article, or any drug-related issue, please contact me at firstname.lastname@example.org In my next article, I will discuss drug effects in the aging body.
Dr. Conte is currently a member of the American Society of Health-Systems Pharmacists and the senior consultant for Rx Conte Consulting. He resides in Aptos, CA and La Conner, WA.