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Prescription Safety: The Most Common Drug Interactions to Know

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Prescription safety is getting more complicated every day. If you’re concerned about your prescription safety, check out these common drug interactions you should definitely know about.

Having a list of prescription drugs in your arsenal can be complicated.

You have to keep track of the proper dose of medicine, when to take each, what foods you can and can’t have with them, what needs to be taken alone, and what needs to be taken with food.

The reality is that this is only half of the battle.

Your pharmacist is your first line of defense when it comes to prescription safety. He or she sees your medications, including what you are filling or taking at the same time, and is most likely to be the first person to catch the potential for common drug interactions.

What Are Common Drug Interactions?

If a doctor or pharmacist prescribes or recommends certain medications, it’s important to follow these orders.

But, have you ever thought about the potential common drug interactions that may happen when you do?

No? Many people don’t.

When you take two or more drugs, there is always the potential for the drugs to interact with each other.

Sometimes, these drugs may not work together and may end up making you feel even worse.

To help you avoid this, take a look at this list of the most common drug interactions that we see.


1. Digoxin and Quinidine

Digoxin is a standard heart medication, often used to treat afib, congestive heart failure, and supraventricular tachycardia; sometimes in combination with a diuretic. Quinidine is also used to address heart rhythm issues but can also be used to treat malaria.

Taken together at regular doses, the levels of digoxin in blood plasma are known to increase significantly, doubling in rate within as little as 24 hours. Patients taking both drugs at the same time may become nauseated, begin vomiting, develop kidney issues, or even die.

In some cases, taking both drugs may be indicated, but if you take them together, you should be carefully monitored for blood plasma levels to prevent issues.

When this combination is indicated, the digoxin dose is usually cut in half compared to what would be taken if alone.

2. Phenelzine and Fluoxetine

Phenelzine is an antidepressant that falls into the MAOI (monoamine oxidase inhibitor) category. Fluoxetine is a similar drug used to treat depression, OCD, some forms of anxiety, and even eating disorders.

Taken together, these drugs can cause a condition known as serotonin syndrome. The symptoms include sudden mental status alterations, a rapid heart rate, and in some cases death. Symptoms can develop after just one or two doses.

In general, MAOIs should not be taken with any drug that can increase your serotonin levels (SSRIs or SNRIs). You should also wait anywhere from two to five weeks after stopping medication in one category before starting a drug in the other category.


3. Warfarin

Patients who are on blood thinners like warfarin should avoid other blood-thinning medications at all cost. Most are told to avoid over-the-counter pain relievers like Aspirin or other drugs containing acetaminophen or NSAIDs.

The problem is that these drugs can be found within other prescription medications, especially pain relievers.

Patients who take these pain relievers with warfarin usually see an increase in their INR of up to 6.0 or higher in as little as a few regular doses. The therapeutic range for INR usually falls between 2.0 and 3.0.

When it exceeds this rating, it significantly increases the risk of bleeding internally as a result of even a mild trauma. Conventional antibiotics also have a similar impact on INR.

Patients who are prescribed warfarin should be counseled by their doctors regarding the use of over-the-counter pain relievers. Pharmacists should also advise patients who pick up warfarin prescriptions. We are often on the lookout for antibiotics or pain relievers that could also impact a patient’s INR.


4. Pseudoephedrine and Prescription Drugs

Drugs that fall into the “Sudafed” cold-medicine category (containing the drug pseudoephedrine) can interact or even contraindicate with many common prescription medications. This includes medicines such as caffeine, dihydroergotamine, hyoscyamine, methylene blue.

You’re more likely to recognize names from the moderate drug interaction list, including but not limited to albuterol, acetaminophen, codeine, digoxin, fluticasone, guaifenesin, hydralazine, HCTZ, insulin, levothyroxine, metformin, and sotalol.

Exactly how pseudoephedrine effects these other medications depends on the drug. In some cases, it raises blood pressure dangerously.

In other cases, it increases blood serum levels to unsafe values. You should be especially cautious if you have any form of heart disease, diabetes, or glaucoma.


5. Methotrexate and Probenecid

Methotrexate is an immunosuppressive drug that is often used in chemotherapy; often for certain types of cancer and sometimes for severe cases of rheumatoid arthritis and psoriasis. Probenecid is a uric acid reducer that is usually with the treatment of gout.

When taken together, these two drugs cause a patient’s body to over-absorb the methotrexate, with a 200 to 300 percent increase in methotrexate levels in the blood. This will lead to a toxic effect that can completely suppress the already compromised immune system, cause diarrhea and vomiting, and cause kidney failure.

The low dose used for arthritis is not associated with the same high risks as the higher doses of methotrexate used in cancer treatments.

6. Clonidine and Propranolol

Clonidine is most commonly used to treat high blood pressure. There are also some drug formulations for ADHD or cancer pain that contain clonidine as a secondary ingredient. Propranolol is also used for high blood pressure but can sometimes be used for atrial fibrillation, angina, and to prevent migraines.

The combination of the two drugs combined can increase the blood pressure instead of lowering it, and suddenly removing clonidine from the combination treatment can cause a suppression of the sympathetic nervous system and a sudden surge in norepinephrine that the body can’t compensate for because of the propranolol.

These common drug interactions cause severe, potentially-fatal rebound hypertension.



Carry a complete medication list with you at all times, especially if you see private practice doctors or doctors who are not all affiliated with the same hospital system. Their systems do not share information.

It will be your responsibility to make sure your medication list is up to date everywhere you go.

Do not be afraid to insist that the nurse or doctor go over your list with you to discuss potential common drug interactions before any changes or additions to your treatment plan. Never hesitate to ask your pharmacist questions if you aren’t sure if the medications you are taking are safe together.

You should also always feel free to take over-the-counter medicines to the pharmacy counter so that you can ask about interactions with your prescription drugs. Your pharmacist will gladly look up your current medications and help you to cross-reference medicines for common drug interactions.

It is not uncommon for a pharmacist to pick up on a medication interaction that a doctor may not have considered.

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