Drug Interactions: What to Watch For
Drug-drug, drug-food, and drug-supplement interactions explained in plain language — the most common dangerous combinations, and how our pharmacists screen every prescription for you.
Why Interactions Happen
Every medication you take changes how your body absorbs, processes, or clears other substances. When two or more medications, foods, or supplements interfere with each other, the result is a drug interaction — and it can make a medication less effective, more dangerous, or both.
The more medications you take, the higher the risk. This is one of the main reasons pharmacists exist as a safety check between your prescriber and the medication in your hand.
Common Types of Interactions
Two prescription or OTC medications that amplify or cancel out each other's effect — e.g. combining blood thinners with certain pain relievers.
Grapefruit juice is the best-known example — it can raise blood levels of many statins and blood pressure medications to unsafe levels.
St. John's Wort, fish oil, and even everyday vitamins can interact with prescription medications in ways many patients don't expect.
Some medications are unsafe for people with specific conditions — kidney disease, glaucoma, or certain heart rhythms — regardless of what else they're taking.
Combinations Worth Knowing About
- ✓Blood thinners (warfarin, DOACs) + NSAIDs — significantly increased bleeding risk
- ✓MAOIs + decongestants or certain antidepressants — can cause a dangerous spike in blood pressure
- ✓Statins + grapefruit juice — can raise drug levels and increase risk of muscle damage
- ✓Benzodiazepines + opioids — combined sedation and slowed breathing, a leading cause of preventable overdose
- ✓ACE inhibitors + potassium supplements — risk of dangerously high potassium levels
How We Screen Every Prescription
Every time you fill a prescription at Tartan Pharmacy, our dispensing software cross-checks it against everything else on your profile — prescription and over-the-counter — and flags any known interaction before it reaches you. But software can only see what's on file.
The single biggest thing you can do to stay safe is make sure we know everything you take, including vitamins, herbal products, and anything you buy at another pharmacy.
When to Ask Your Pharmacist
- ✓Before starting any new over-the-counter medication or supplement
- ✓If a new prescriber adds a medication without seeing your full list
- ✓If you notice a new symptom shortly after starting or combining medications
- ✓Anytime you're not sure — there's no such thing as a bad question at the counter
Have a question about your medications?
Our pharmacists are available to talk through anything in this article, in person or over the phone — no appointment necessary.
Nepean, ON K2J 2W7613-823-8030Mon–Fri 9 AM–5 PM
Sat–Sun Closed
Related Guides
Understanding Your Blood Pressure Medications
A plain-language guide to the main classes of blood pressure medications — how they work, common side effects,…
Read Guide →Antibiotics: What You Need to Know
What antibiotics actually treat, why finishing the full course matters, and how antibiotic resistance affects …
Read Guide →Medical Disclaimer: The information in this article is intended for general informational purposes only and does not constitute medical advice. Always consult your pharmacist or physician before making any changes to your medications, treatments, or health routine.