I have a cold. I want to take over the counter medications and vitamin C. Do I need to ask my doctor first? Will cough medicine affect my prescription medications?

Whether you have a cold, want to take an Over-The-Counter (OTC), or herbal product for any reason, drug interactions are a concern, especially when you already use prescriptions drugs.

There are three basic kinds of drug interactions: Drugs interacting with other drugs; drugs interacting with food or beverages; and drugs interacting with chronic conditions you already have, that may not even be related to the interacting drug you want to take. OTC drugs, vitamins, and herbals all act like drugs and potential drug interactions need to be monitored. Some drug interactions can be beneficial but many more can be dangerous.

For that reason, many drug to drug interactions need to be avoided. The herbal product St. John’s Wort has actions similar to antidepressants so some people like to take it for that reason. The problem is that this herbal remedy interacts with prescription antidepressants and can cause very serious reactions. Avoid the use of these two together unless a provider says to take both. In another case of drug interactions, timing is everything. As an example, a person taking thyroid medicine should avoid other drugs at the same time, including vitamins: To do that, the thyroid prescription is taken early in the morning on an empty stomach and the vitamin is taken an hour later with breakfast, avoiding the interaction and allowing both drugs to be taken.

Drug interactions with food or beverages sometimes go beyond a simple dietary choice. For example, there is an antibiotic tablet, metronidazole, which reacts significantly with alcohol. Even if you say you don’t drink, your body doesn’t know the difference of this interaction with alcohol from cough syrup, herbal liquid supplements, or even some kinds of candies, and rum cake. This unpleasant combination of drug and alcohol can cause violent vomiting and a bright red hot flushing of the face and torso. Another example of the need to avoid certain food or beverages with a drug is due to the fact that alcohol is known for deadly reactions with narcotics and sedatives. Many deaths have occurred from what the small label on your prescription warns: “Use of alcohol may intensify the effect of this drug” followed by cautions about driving impaired or using machinery.

Sometimes drug-disease interactions occur because of multiple ingredients in the drug. For example, the active or main ingredient of a cough syrup may be fine with a prescription to improve control of blood sugar, but the extra ingredients of alcohol or sugar in the cough syrup can cause problems with that chronic disease, diabetes. Vitamin C can affect blood sugar too and has other interactions with several drugs that affect blood clotting.

There are likely more drug interactions possible than there are drugs. Read labels and whether you want to take something for relief or comfort for a temporary situation, note that there are many drugs with significant interactions. The best advice is to contact your pharmacist to review your prescription profile and receive well researched and practical counseling before taking any OTC drugs, vitamins, or herbals.

Janet Schade, MS, RPh

Disclaimer: This column is not intended as a diagnosis or recommended treatment of a specific condition. Answers are not a replacement for an individual medical evaluation. Individual health concerns should be evaluated by a licensed clinician.