Almost every general sale and pharmacy-only medicine carries a small set of well-described interactions. Most are mild. A few — anticoagulants and ibuprofen, decongestants and certain antidepressants, antacids with thyroxine — can cause serious harm. The community pharmacist is often the only healthcare professional who sees the purchase before it is taken.

This guide collects the over-the-counter interactions that most often need an intervention at the till, drawn from the British National Formulary (BNF), the NICE Clinical Knowledge Summaries (CKS) and Medicines and Healthcare products Regulatory Agency (MHRA) safety alerts.

NSAIDs are the highest-yield safety check

Ibuprofen, naproxen and aspirin appear in dozens of OTC analgesic, cold and flu and topical preparations. Three interactions matter most.

The MHRA issued a long-standing reminder that occasional NSAID use in patients on anticoagulants should be discussed with the prescriber rather than sold over the counter.

Antacids and absorption

Aluminium-, magnesium- and calcium-containing antacids (Gaviscon, Rennie, Maalox) bind several commonly prescribed medicines and reduce their absorption. The standard advice is to leave at least four hours between the antacid and the affected medicine where possible.

• Levothyroxine — separation by four hours; otherwise thyroid function may drift.
• Tetracyclines and quinolones (doxycycline, ciprofloxacin) — chelation reduces antibiotic levels significantly.
• Bisphosphonates (alendronic acid) — already require strict empty-stomach dosing.
• Iron salts — significantly reduced uptake.
• Some antiretrovirals — refer to specialist before recommending OTC antacids.

PPIs and clopidogrel

Omeprazole and esomeprazole reduce the antiplatelet effect of clopidogrel through CYP2C19 inhibition. The MHRA advises avoiding the combination where possible. When a patient on clopidogrel asks for OTC heartburn relief, pantoprazole, lansoprazole or an H2 blocker (famotidine) are usually safer alternatives.

Decongestants and antidepressants

Pseudoephedrine, phenylephrine and ephedrine — found in cold and flu remedies such as Sudafed and many own-label products — interact with two antidepressant classes.

• Monoamine oxidase inhibitors (phenelzine, tranylcypromine, moclobemide, selegiline): hypertensive crisis risk. Avoid for two weeks after stopping an MAOI.
• Some serotonergic combinations have rarely been associated with a serotonin syndrome–like presentation.

Decongestants are also contraindicated in uncontrolled hypertension, hyperthyroidism and severe coronary disease. If a patient mentions any of these, switch to a saline nasal spray or steam inhalation.

St John's wort

The herbal antidepressant is a potent inducer of cytochrome P450 enzymes and reduces plasma levels of many medicines, including:

• combined oral contraceptives (risk of unintended pregnancy)
• warfarin (loss of anticoagulation)
• ciclosporin and tacrolimus (transplant rejection risk)
• digoxin
• many HIV protease inhibitors and integrase inhibitors
• triptans (additive serotonergic effect)

The BNF advises against self-supply of St John's wort to anyone on these classes.

Loperamide

The OTC anti-diarrhoeal has been associated with QT prolongation and ventricular arrhythmia at high doses. The MHRA highlighted misuse cases in 2017 and reiterated dose limits: maximum 16 mg in 24 hours for adults using OTC product. Counter staff should refer cases of repeat purchase or unusually large-quantity sales.

Codeine-containing analgesics

Co-codamol, Solpadeine, Nurofen Plus and similar combinations carry the standing MHRA restrictions: short-term acute pain only, three days maximum, with addiction warnings on the pack. Concomitant alcohol, sedating antihistamines, gabapentinoids or other opioids should be flagged. Patients on CYP2D6-inhibiting antidepressants (paroxetine, fluoxetine, bupropion) may experience reduced analgesic effect because codeine is converted to morphine via that pathway.

Statins and grapefruit

Simvastatin and atorvastatin are metabolised by CYP3A4. Grapefruit juice, particularly in regular intake of more than 200 ml per day, raises plasma statin levels and the risk of myopathy. The interaction is most marked for simvastatin. Pravastatin and rosuvastatin are minimally affected.

Combined oral contraceptives

Vomiting within two hours of a pill, severe diarrhoea for more than 24 hours, and short courses of enzyme-inducing drugs all reduce contraceptive effectiveness. Pharmacy customers buying loperamide, anti-emetics, or starting modafinil or rifampicin should be reminded to use additional contraception.

When to refuse the sale

A clear refusal is sometimes the safest intervention. The MHRA, the General Pharmaceutical Council (GPhC) and individual professional bodies all support pharmacists declining a sale where the risk of harm outweighs the benefit. Common refusal scenarios include:

• NSAIDs requested by a patient already on anticoagulants
• Pseudoephedrine for a patient with poorly controlled cardiovascular disease
• Repeat large-volume purchases of codeine combinations
• Loperamide purchases that suggest stockpiling
• Any combination flagged in the BNF as severe risk

A short, courteous explanation and a documented signposting to the GP usually preserves the patient relationship and the pharmacy's professional reputation.

Building counter-team confidence

PharmSee's pharmacy salary data shows that medicines counter assistants and dispensers make up the majority of frontline community pharmacy roles. Their training in interaction recognition is the first defence against the sales above. Practical steps include:

• Laminated counter cards listing the top ten interaction pairs
• Regular short refreshers using BNF Appendix 1 worked examples
• A documented pharmacist-on-call escalation pathway for any borderline sale
• A no-blame review of any near-miss caught at point of sale

The PharmSee jobs board lists current pharmacy team vacancies across the UK; the pharmacist career hub covers the wider clinical responsibilities of the medicines safety role.

Sources

• British National Formulary, Appendix 1: Interactions (2026 edition)
• NICE Clinical Knowledge Summary: NSAIDs prescribing issues
• MHRA Drug Safety Update — multiple bulletins on PPI/clopidogrel, loperamide misuse, NSAIDs and anticoagulants
• General Pharmaceutical Council guidance on professional judgement in sales of medicines