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Drug Interactions: What You Need to Know

By Evan Starkman

Medically Reviewed by Joshua Conrad, PharmD

What Are Drug Interactions?

When a medication works right, it boosts your health or helps you feel better. But a drug can bring on problems if it doesn't mix well with something else you put into your body, like another medication, a certain food, or alcohol.

When that happens, it's called a drug interaction. It could make your medication stop working, become less effective, or too strong. It could also trigger side effects.

The more you learn about drug interactions, the better you'll be able to avoid them. Here's what you need to know.

What Are the 3 Types of Drug Interactions?

The main types are:

Drug-drug interaction. This is when a medication reacts with one or more other drugs. For example, taking a cough medicine (antitussive) and a drug to help you sleep (sedative) could cause the two medications to affect each other.

Drug-food/drink interaction. This is when something you eat or drink affects a drug. For instance, it can be dangerous to drink alcohol while you're on certain medications. Some vitamins and dietary supplements interact with medicines, too.

Drug-condition interaction. This is when you have a health problem that makes it risky for you to take certain meds. For example, if you have a condition like high blood pressure, taking a decongestant for a cold could drive up your blood pressure even more.

What Drugs Can You Not Take Together?

There are many types of drugs you shouldn't take together, but in general, don't take combinations like these:

Two or more drugs that share an active ingredient. You could have side effects or an overdose. Active ingredients are the chemicals in medications that treat your condition or symptoms. Always check for them on the drug label.

Blood-thinning drugs with NSAIDs. Your odds for a dangerous bleed could go up. NSAIDs (nonsteroidal anti-inflammatory drugs) are pain relievers like ibuprofen or naproxen. If you're on a blood thinner, ask your doctor to suggest a different type of over-the-counter pain medication and dose that's safer for you.

Pills with antihistamines. Taking these together can cause you to react more slowly, which would make it dangerous for you to drive or work with heavy machines.

Some other drug-drug combinations that can be dangerous are:

  • An NSAID pain reliever with a blood pressure drug
  • A thyroid drug called levothyroxine with an acid-blocking proton pump inhibitor drug called omeprazole
  • An SSRI antidepressant with another medicine that affects serotonin (such as dextromethorphan, linezolid, tramadol, and trazodone)
  • A cholesterol-lowering statin with antifungal and fibrate meds that are commonly called "azole" drugs (Their technical names are "imidazole and triazole derivatives." They lower blood fats called triglycerides.)
  • The antibiotic clarithromycin with a type of blood pressure drug called a calcium channel blocker

What Are Common Drug Interactions?

Certain foods and drinks don't mix well with some medications. A few of these are:

Alcohol. Booze can bring on dangerous side effects with many medications, including some drugs for:

Before you start a new medication, ask your doctor or pharmacist if it could interact with alcohol. Check the drug label for alcohol warnings, too.

Grapefruit or grapefruit juice. Too much of either can affect some drugs like:

Grapefruit juice doesn't mix badly with every type of drug in these classes of medications. Check your drug's label or information pamphlet for any warnings about it. Also, ask your doctor or pharmacist if it's safe to have grapefruit or its juice in any amount with your specific drug. If they tell you to stop eating or drinking it, ask if any other fruits or juices might have similar effects on your medicine.

Foods with vitamin K, like leafy greens. These can interact with the blood thinner warfarin. You don't have to stop eating them, but it's important to be consistent and not overdo it. Ask your doctor how much food with vitamin K you can have, then eat the same amount around the same times each week.

High-potassium foods and drinks, like bananas, salt substitutes, and orange juice. These can affect blood pressure drugs called ACE inhibitors. Your doctor will track your potassium levels, and they may tell you to cut back on foods with it.

St. John's wort. This herbal dietary supplement can affect many medications for heart disease, HIV, depression, and other conditions. It can also affect birth control pills and the cancer drugs irinotecan and imatinib.

Always get your doctor's or pharmacist's OK before you try a new supplement. It's also possible for a medication you take to interact with a health condition you have. Some common drug-condition interactions happen between:

Is It OK to Take All Your Medications at Once?

It's important to take your medication as directed by your doctor or pharmacist. If two meds interact, spacing them apart during the day may not stop them from interacting and could lead to new or worse problems. To avoid an interaction, your doctor may need to change your dose or prescribe a different medication.

What Are the Symptoms of a Drug Interaction?

The symptoms of a drug interaction can vary a lot, depending on the drugs you’re taking and how they’re interacting. Sometimes you might not even know right away that an interaction is happening.

If you do notice an interaction, it will usually feel like one of these is happening:

  • You are having more side effects from a drug
  • Your drug doesn’t seem to be working as well as it was before

In general, call your doctor if you're having side effects that worry you or if your medicine doesn't seem to be working, especially after you’ve recently started or changed doses of a drug.

How Do Drug Interactions Happen?

There are several ways that drugs can interact with one another. Here are some examples of the most common ways.

When two drugs can cause the same side effect and are used at the same time, they might cause more of that side effect. For example, if two drugs can each make you sleepy, taking them together can make you more or dangerously sleepy.

When the unwanted effects of one drug are the opposite of the desired effects of another drug, you might end up with less of the desired effects. For example, taking one medication that raises blood pressure as an unwanted effect may decrease the benefits of taking another medication to lower your blood pressure.

Most drugs that you swallow enter your blood through your intestines. Sometimes a drug or supplement can block or trap another drug in the intestine before it can be absorbed. For example, supplements like calcium and iron can prevent absorption of thyroid meds.

Your body has enzymes, such as the cytochrome p450 (CYP) and others, that process many types of medications. This is called metabolism. It also has a way to get rid of drugs, usually though your urine. Other drugs may speed up, slow down, or even completely block these functions. When this happens, the amount of drug in your body may increase (similar to taking too much) or decrease (similar to taking too little). Either way, this could cause serious problems.

How Do You Look for Drug Interactions?

Work closely with your doctor. Make sure they know all the medicines, vitamins, and supplements you're taking. That's extra important if you have more than one doctor who prescribes medicines for you. You could show them a list of the meds you're taking, or bring the medication packages to your appointment.

Before you take a new drug, ask your doctor or pharmacist questions like:

  • Can I take this with my other meds?
  • Should I avoid certain foods, drinks, or other products?
  • Could this medication interact with any health conditions I have?

Pharmacists are experts on medicine safety, and they can work with your doctors to help you avoid drug interactions. For example, if you have two doctors and they separately prescribe drugs that interact, your pharmacist can warn them -- and you -- before you have a problem.

Read labels carefully. Over-the-counter drug labels include information about possible drug interactions and the medication's active ingredients. Prescription medications usually come with a sheet that explains what the drug is and how to take it safely.

But most drug labels and patient handouts don't list every possible drug interaction. Talk to your pharmacist to get the full picture. They can also answer any questions about medical terms or jargon on the drug packages.

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