Drug Interactions: What to Watch For
Understanding how your medications interact with other drugs, foods, and supplements is an important part of staying safe. Your pharmacist is your best resource for catching potential problems before they occur.
Types of Drug Interactions
A drug interaction occurs when a substance changes the way a medication works in your body. Interactions can make a drug more or less effective, or increase the risk of side effects. There are three main types:
Drug–Drug
One medication affects how another is absorbed, broken down, or eliminated. This is the most common type.
Drug–Food
Certain foods and beverages change drug levels in your blood. Grapefruit is the most well-known example.
Drug–Supplement
Natural health products, vitamins, and herbal remedies can interact with prescription and OTC drugs.
Common Interactions to Be Aware Of
The following are some of the most clinically significant interactions seen in community pharmacy practice. This is not an exhaustive list — always ask your pharmacist about your specific medications.
Warfarin + ASA (Aspirin)
Risk: Significantly increased bleeding risk, including potentially serious internal bleeding.
What to know: Sometimes this combination is intentional (e.g., after a heart stent) but requires close monitoring. Never start ASA while on warfarin without medical guidance.
SSRIs + NSAIDs (e.g., ibuprofen, naproxen)
Risk: Combination increases the risk of gastrointestinal bleeding significantly compared to either drug alone.
What to know: People on antidepressants (SSRIs like sertraline, escitalopram) should use acetaminophen for pain when possible.
Grapefruit + Certain Statins
Risk: Grapefruit can raise blood levels of simvastatin and lovastatin to potentially toxic levels, increasing the risk of muscle damage (rhabdomyolysis).
What to know: Atorvastatin is affected to a lesser degree. Pravastatin and rosuvastatin are generally safe with grapefruit.
St. John's Wort + Multiple Medications
Risk: This herbal supplement speeds up the breakdown of many medications, making them less effective. Affected drugs include oral contraceptives, warfarin, antiretrovirals, and some antidepressants.
What to know: St. John's Wort can also cause serotonin syndrome when combined with SSRIs or SNRIs.
ACE Inhibitors/ARBs + Potassium Supplements or NSAIDs
Risk: Can lead to dangerously high potassium levels (hyperkalemia) or reduced kidney function.
What to know: Avoid potassium supplements or salt substitutes without medical advice if you are on these blood pressure medications.
Supplements That Can Interact With Medications
Many people assume that natural health products are safe to take alongside prescription medications. In reality, supplements can have significant interactions. Here are some common ones to discuss with your pharmacist:
- St. John's Wort (Hypericum): speeds up the breakdown of many medications including warfarin, oral contraceptives, and antidepressants. Can also trigger serotonin syndrome with SSRIs.
- Fish oil / Omega-3 fatty acids: at high doses, may increase bleeding risk, especially with blood thinners.
- Vitamin K: found in leafy greens and some supplements — reduces the effectiveness of warfarin. Consistency of intake is important, not elimination.
- Calcium: can reduce the absorption of thyroid medications (levothyroxine) and some antibiotics (quinolones, tetracyclines). Take at least 2–4 hours apart.
- Magnesium and antacids: can reduce absorption of certain antibiotics and other medications.
- Melatonin: may interact with blood pressure medications and blood thinners.
What to Tell Your Pharmacist
Your pharmacist can only screen for interactions they know about. To get the most complete protection, share the following every time you start a new medication:
- All prescription medications from every doctor or specialist you see.
- Over-the-counter medications including pain relievers, antacids, antihistamines, and sleep aids.
- All vitamins, minerals, and supplements — including those purchased outside a pharmacy.
- Any herbal or traditional medicine products.
- Whether you smoke, drink alcohol, or use recreational substances.
- Any known drug allergies or previous adverse reactions.
Frequently Asked Questions
How does my pharmacist check for drug interactions?
Every time you fill a prescription at Longfields Pharmacy, our pharmacy software automatically screens your full medication list for potential interactions. Our pharmacists also perform manual clinical reviews. This is why it is important to fill all of your prescriptions at the same pharmacy — we can only screen for interactions with medications on your profile.
Are natural health products and vitamins safe to take with my medications?
Not always. Many natural health products, herbal supplements, and vitamins can interact with prescription medications. Common examples include St. John's Wort (interacts with antidepressants, birth control, warfarin), fish oil at high doses (can increase bleeding), vitamin K (interferes with warfarin), and calcium (can reduce absorption of certain thyroid medications and antibiotics). Always tell your pharmacist about every supplement you take.
Why does grapefruit interact with so many medications?
Grapefruit contains compounds called furanocoumarins that block an enzyme in your gut (CYP3A4) responsible for breaking down many medications. When this enzyme is inhibited, drug levels in your blood can rise to unexpectedly high levels — increasing the risk of side effects. The effect can last up to 72 hours from a single glass of grapefruit juice. Affected medications include certain statins, some calcium channel blockers, and several others.
What information should I give my pharmacist to help prevent interactions?
Tell your pharmacist about every prescription medication you take (from all doctors), all over-the-counter drugs including pain relievers, antacids, and sleep aids, all vitamins and supplements, any herbal or natural health products, and whether you smoke or drink alcohol regularly. Even medications prescribed by specialists (such as a cardiologist or psychiatrist) should be on your profile.