Drug interactions: how medicines, food and supplements can clash

1. Introduction

When you take more than one medicine, or take a medicine with certain foods or drinks, they may affect each other. This is called a drug interaction. The US Food and Drug Administration (FDA) explains that an interaction can make a medicine work less well, cause an unexpected side effect, or make an effect stronger, and that some interactions can be harmful ^[1]. This article explains the types of interaction and how to lower your risk. It is education only, and your doctor decides what is right for you.

2. Types of interaction

The FDA describes three main types ^[1]:

• Drug-drug interaction. Two or more medicines react with each other. For example, a sedative taken with an antihistamine can slow your reactions, which makes driving or using machinery dangerous.
• Drug-food or drink interaction. A medicine reacts with something you eat or drink. Alcohol with some medicines can cause tiredness or slowed reactions.
• Drug-condition interaction. A health condition you already have makes a medicine risky. For example, a nasal decongestant may cause an unwanted reaction in someone with high blood pressure.

A sedative is a medicine that causes calm or sleepiness, and an antihistamine is a medicine used for allergy symptoms. A decongestant is a medicine that relieves a blocked nose.

3. Who is more at risk

The FDA notes that older adults often take more prescription and over-the-counter medicines, which raises the chance of interactions and side effects ^[2]. The FDA adds that knowing what you take matters more when you use several medicines, see more than one doctor, or have certain health conditions ^[1]. Interactions can arise when a condition makes a medicine risky, when two medicines affect each other, when herbal products or supplements interfere with a medicine, or when food or drink changes how a medicine is absorbed ^[2].

4. Everyday examples

The FDA's guidance on over-the-counter warnings gives many examples ^[1]:

• Allergy medicines, cough medicines and night-time sleep aids carry warnings about taking them with sedatives, and some say to avoid alcohol. Pain relievers each carry their own warnings too.
• Nasal decongestants and some bronchodilators (medicines that open the airways) call for a check with a doctor first if you have heart disease, high blood pressure, thyroid disease or diabetes.
• Several acne products used together can increase dryness or irritation, so use only one unless your doctor says otherwise.
• Stimulants may contain about as much caffeine as a cup of coffee, so limit caffeine from other sources.

The FDA also points out that grapefruit juice is a common example of a food that can interact with certain medicines ^[2]. Some medicines should not be combined with alcohol, which can cause poor coordination, memory problems, drowsiness and falls ^[2].

The same ingredient twice

Watch for the same active ingredient in more than one over-the-counter product, because this can lead to taking too much without realising it ^[1]. The CDC gives the same advice for children: check active ingredients and do not give two products that contain the same one ^[3].

5. How to lower your risk

• Read the label every time you use any medicine, because labels can change when new information appears ^[1].
• Tell your doctor about all medicines you take, including over-the-counter drugs, vitamins, minerals, herbal products and your diet ^[1].
• Keep a written list with names, amounts and timing, and show it to every provider ^{[2, 3]}.
• Ask your pharmacist for the package leaflet for each prescription medicine ^[1].
• Before starting a medicine, ask whether it can be taken with your other medicines, which foods or products to avoid, what interaction signs to watch for, and how the medicine works ^[1].
• Talk to your doctor before taking medicine if you are pregnant or breastfeeding ^[1].
• Know the ingredients of your medicines to avoid allergic reactions ^[1].

Reading an over-the-counter label

The FDA explains that an over-the-counter label sets out the active ingredients and their purpose, the uses, the warnings, the directions, other information, the inactive ingredients and a number for questions ^[1]. The warnings tell you when to speak to a doctor or pharmacist, when not to use the product, and when to stop. The directions tell you how much to use and for how long. The FDA stresses that examples of warnings in its guidance do not cover everything and do not replace the product label ^[1].

6. When to see a doctor

Speak to your doctor or pharmacist before combining any new medicine, supplement or herbal product with your current ones. Call them if you notice new drowsiness, dizziness, confusion or any unusual symptom after a change. Side effects such as memory trouble, dizziness or sleepiness can mimic other health problems, so ask whether a medicine could be the cause ^[2].

In an emergency, call your local emergency number (112 in India, 911 in the United States). Seek urgent help for swelling of the face or throat, trouble breathing, fainting or severe confusion.

7. Conclusion

Interactions can happen between medicines, between a medicine and food or drink, and between a medicine and a health condition. The most useful steps are simple: read labels, keep a full list, tell every provider about everything you use, and ask your pharmacist. Traditional and herbal treatments should be included in that conversation.