How to Check for Drug Interactions, Especially When You Shop Around

Price shopping can save hundreds of dollars a year, but it can also split your records across several pharmacies. Here is how to keep the savings without missing a dangerous interaction.

The Short Answer

A drug interaction happens when one substance changes how a medicine works in your body. The other substance can be a second medicine, a supplement, a food, a drink, or even a health condition. Some interactions make a medicine weaker. Others make it stronger or cause new side effects.

Your pharmacist is your best free safety check, but only if the pharmacist sees your full medicine list. If you fill prescriptions at more than one pharmacy to get lower prices, carry an up-to-date list and give it to every pharmacist. That one habit lets you keep the savings and the safety.

The Three Types of Drug Interactions

The FDA groups drug interactions into three broad types.

1. Drug and Drug

Two or more medicines react with each other. The FDA gives this example: a sleep aid taken with an allergy antihistamine can slow your reactions and make driving dangerous. Over-the-counter medicines count too. Many people do not think of a pain reliever or a cold medicine as a "drug," but these can interact with prescriptions.

2. Drug and Food or Drink

A food or drink changes how a medicine works. Alcohol with some medicines can cause extra drowsiness or slowed reactions. Grapefruit and grapefruit juice can raise the level of some medicines in the blood, including certain cholesterol and blood pressure drugs. Check your medicine's label or ask your pharmacist whether a food matters for you.

3. Drug and Condition

A health condition makes a medicine risky. The FDA notes that a person with high blood pressure may react badly to a nasal decongestant. This type of interaction is easy to miss on a price comparison site, because the site does not know your health history. Your doctor and pharmacist do.

Supplements Count Too

Vitamins, herbal products, and other supplements can interact with prescription medicines. For example, St. John's wort can make several medicines less effective. Many people leave supplements off their medicine list because they bought them without a prescription. Put every supplement on your list, with the dose and how often you take it.

Read the Drug Facts Label

Every over-the-counter medicine has a Drug Facts label. The FDA points to these sections as the most important for safety.

• Active Ingredients: the medicine inside and the amount. Two different products can contain the same active ingredient. Taking both can mean a double dose.
• Uses: the symptoms or conditions the product treats.
• Warnings: when to ask a doctor or pharmacist first, which conditions make the product unsafe, and when to stop.
• Directions: how much to take and how often.

Acetaminophen is the classic double-dose risk. It appears in many pain relievers, cold medicines, and sleep aids. Read the active ingredients on every product you take together.

Why Using Several Pharmacies Raises the Risk

Each pharmacy's computer checks new prescriptions against the medicines in its own records. If you fill your blood pressure medicine at one store and a new antibiotic at another, neither system sees both drugs. The pharmacist at the second store cannot warn you about a combination it does not know about.

That does not mean you should stop comparing prices. The price difference for the same drug can be large. It means you need to close the information gap yourself.

Five Steps to Stay Safe and Save Money

1. Keep one complete medicine list. Include every prescription, over-the-counter medicine, vitamin, and supplement. Write the name, the dose, and how often you take it. Keep a copy on your phone and in your wallet.
2. Hand the list to the pharmacist at every new pharmacy. Ask the pharmacist to add your other medicines to your profile. Most pharmacies will record medicines filled elsewhere.
3. Ask three questions with every new prescription. Can I take this with my other medicines? Should I avoid any foods, drinks, or supplements? What warning signs should I watch for?
4. Use a free interaction checker as a second look. MedlinePlus, from the National Library of Medicine, has plain-language drug information. Several drug reference sites offer free interaction checkers. A checker does not replace your pharmacist, but it can prompt a good question.
5. Bring your list to every doctor visit. Specialists may not know what your other doctors prescribed. A complete list helps every prescriber choose safely.

Warning Signs to Take Seriously

Call your doctor or pharmacist if you notice new symptoms after you start a medicine. Common signs of an interaction include unusual drowsiness, dizziness, a racing or slow heartbeat, unusual bleeding or bruising, or a medicine that suddenly stops working. Call 911 for trouble breathing, chest pain, fainting, or swelling of the face or throat.

Do not stop a prescribed medicine on your own because you suspect an interaction. Some medicines are dangerous to stop suddenly. Call your prescriber or pharmacist first.

A Simple Medicine List Template

Add a line for allergies and for any medicine you stopped in the past year. Update the list every time a doctor changes a dose.

Frequently Asked Questions

Does the pharmacy check interactions automatically?

Yes, for the medicines in its own records. Pharmacy software flags known interactions when it processes a new prescription. The check cannot include medicines filled at another pharmacy, bought over the counter, or taken as supplements unless you tell the pharmacist about them.

Is it safe to use different pharmacies for different medicines?

Yes, if you keep every pharmacist informed. Give each pharmacy your complete medicine list, and ask each one to add your outside medicines to your profile. Update the list whenever a dose changes.

Do online interaction checkers replace my pharmacist?

No. Online checkers list known interactions between drug pairs. They do not know your age, kidney function, other conditions, or how long you have taken a medicine. Use a checker to prepare questions, then talk with your pharmacist or doctor.

Should I worry about every interaction a checker shows?

Not always. Many listed interactions are minor, and doctors often prescribe certain combinations on purpose with extra monitoring. Do not stop a medicine because of a checker result. Ask your prescriber or pharmacist what the warning means for you.

How often should I review my medicines?

Review your full list with your doctor or pharmacist at least once a year. Many pharmacies offer a free medication review. People with Medicare Part D who take several medicines may qualify for a free Medication Therapy Management review through their plan.

What should I bring to a medication review?

Bring every medicine you take, in its original container, including supplements and over-the-counter products. Bring your written list and the names of all your doctors and pharmacies.

The Bottom Line

Comparing prices and staying safe can work together. Keep one complete medicine list, give it to every pharmacist and doctor, and ask about interactions with every new prescription. Your pharmacist can check for interactions for free, but only with the full picture.