What this means in practice
Reading the active ingredient on a second pain, cold or flu product before taking it alongside the first is the whole of it. For most people nothing changes, because most people take one product at a time without ever facing the choice.
What people say
They are different painkillers, so you can take one for your back and the other for your headache. And if one is not touching it, adding the other should help.
What the evidence supports
The behaviour is common and largely unrecognised by the people doing it. In a one-week diary of 1,326 ibuprofen users, 37% also took a second, different non-steroidal anti-inflammatory product, and most did not recognise every product they were taking as one; 11% went over the daily limit for ibuprofen on its own (Kaufman et al., 2018). What the stacking costs is established at the level of dose rather than for this specific pair: pooled observational data put high daily doses at two to three times the serious upper gastrointestinal risk of low daily doses (Castellsague et al., 2012), and pooled randomised data put ibuprofen and naproxen at the top of the class for that harm at high doses, near four times placebo each (Coxib and traditional NSAID Trialists' Collaboration, 2013). No trial has tested this combination against either drug alone, so the harm side is inferred from the dose-response curve rather than measured for the pair.
How it is supposed to work
Both drugs relieve pain by blocking the same cyclo-oxygenase enzymes, so a second one does not open a new route to relief while it does add to the same stomach, kidney and cardiovascular load. The proposed result is more harm at no more benefit.
Which way it runs
Symmetric: neither drug acts on the other, and the order they are taken in does not matter. The risk comes from taking both, and ibuprofen with naproxen is only the most common instance of it. The same relation holds for any two non-steroidal anti-inflammatory drugs taken together, including one hidden inside a multi-ingredient product.
What is still missing
The popular version counts two brand names as two different medicines, where the body counts them as one larger dose of the same thing.
Sources
- Exceeding the daily dosing limit of nonsteroidal anti-inflammatory drugs among ibuprofen usersKaufman DW et al., 2018
- Individual NSAIDs and upper gastrointestinal complications: a systematic review and meta-analysis of observational studies (the SOS project)Castellsague J et al., 2012
- Vascular and upper gastrointestinal effects of non-steroidal anti-inflammatory drugs: meta-analyses of individual participant data from randomised trialsCoxib and traditional NSAID Trialists' (CNT) Collaboration (Bhala N et al.), 2013