Most interaction problems with pharmacy medicines fall into four patterns. Recognising which one you might be creating is more useful than trying to memorise lists.
- Doubling up. Two products containing the same ingredient. Most often paracetamol, sometimes an anti-inflammatory or a decongestant.
- Adding up. Different ingredients with the same effect. Two sedating products cause more drowsiness than either alone.
- Working against. A pharmacy product undoing a prescription medicine, such as a decongestant raising blood pressure that is being treated.
- Blocking absorption. Antacids, calcium and iron binding other medicines in the gut so they are not absorbed.
Common side effects of this pharmacy medicine or category
An interaction often shows up as an ordinary side effect that is unexpectedly severe: more drowsiness than the label suggests, palpitations after a cold remedy, or dizziness after adding something new. If an effect seems disproportionate, a combination is a likely explanation.
Interactions and duplicate-ingredient risks
The combinations that come up most often in practice:
- Anti-inflammatories with blood thinners, steroids or SSRIs. Each combination raises bleeding risk. See NSAID side effects.
- Anti-inflammatories with blood pressure medicines and diuretics. A recognised cause of kidney injury.
- Decongestants with blood pressure medicines, or with stimulants and thyroid medicine. See decongestants and blood pressure.
- Sedating antihistamines with sleeping tablets, opioid painkillers or alcohol. Compounded drowsiness, and impaired driving.
- Antacids, calcium or iron with thyroid medicine or certain antibiotics. Absorption blocked; separate by several hours.
- St John’s Wort with almost anything, including contraception and warfarin. See St John’s Wort interactions.
Supplements count as medicines for this purpose, and are the category people most often forget to mention.
Children, pregnancy, older adults and chronic medicine cautions
Interaction risk rises with the number of products taken, which is why older adults and people on chronic medication are most affected. See OTC medicines in older adults and OTC medicines with chronic medication.
Warning signs and overdose concerns
Emergency care for: vomiting blood or black tarry stools; difficulty breathing or facial swelling; chest pain or a racing heartbeat; very slow breathing or unrousable drowsiness; confusion that is out of character; or any suspected overdose.
Questions to ask a pharmacist before using it
The single most useful habit is bringing a current list of everything you take, including supplements and anything bought elsewhere. A pharmacist can only screen against what they know about.
- Does this clash with anything on my list?
- Does it duplicate an ingredient I already take?
- Does anything need spacing apart by a few hours?
- Will the combination affect my driving?
- Is there a safer alternative given what I already take?
Related OTC and medicine-safety guides
- OTC medicine side effects
- How to read over-the-counter medicine labels
- When to see a doctor about an OTC medicine
- Supplement and medicine interactions
- Report an OTC medicine side effect
Medical Disclaimer: This content is for informational and educational purposes only and does not constitute medical diagnosis, prescription, or professional advice. Always read the patient information leaflet and consult a qualified doctor or pharmacist regarding any medicine or health concern.