Drug interactions in older people: the most common ones
Which medicine combinations cause most problems in older people, which signs to watch for and how to avoid them with a single list and regular reviews.
In short
- An interaction happens when a medicine, food or herbal remedy changes the effect of another medicine.
- Older people are at higher risk because they take more drugs, clear them more slowly and often see several doctors.
- The most dangerous combinations include an anticoagulant with an anti-inflammatory, the "triple whammy" for the kidneys and sedatives with opioids.
- The best prevention is a single medication list reviewed by the doctor and pharmacist, and no self-medication.
The most common drug interactions in older people are those that increase the risk of bleeding, damage the kidneys or cause drowsiness and falls. They can almost always be prevented if someone has the full list of what the person takes in view.
In this article we explain what an interaction is, which combinations are of most concern according to official sources and what you can do as a family member or carer.
| What will you find in this article? |
What is a drug interaction?
An interaction happens when one medicine changes the effect of another: it can strengthen it, cancel it out or add to its side effects. Interactions can also occur with food, alcohol, herbal remedies and with the person's own health conditions.
Medicine to medicine interactions are the best known, but the MSD Manual points to two others that are often overlooked: over the counter products (such as anti-inflammatories or sedating antihistamines for sleep) and herbal supplements, which many people forget to mention to their doctor.
- Strengthening: one drug increases the effect of the other, for example a higher risk of bleeding.
- Cancelling out: one drug reduces the effect of the other, which then stops protecting.
- Adding side effects: two drugs that cause drowsiness or dizziness reinforce each other.
- Drug to disease interaction: a medicine makes another health problem worse.
Why are older people at higher risk?
Because they usually take more medicines at the same time (polypharmacy), clear them more slowly due to changes in the kidneys and liver, and are often prescribed medicines by several different doctors. Every new drug multiplies the possible combinations.
The MSD Manual also describes the prescribing cascade: a side effect is mistaken for a new illness and treated with another medicine, instead of reviewing the one causing it. It warns that hospital discharges and changes of doctor are moments with many errors, such as duplicating the same active ingredient under two different brands.
That is why geriatric specialists use tools such as the STOPP/START criteria, whose version 3 in Spanish (2023) contains 190 criteria to detect potentially inappropriate medication in older people. For more background, see our article with tips for organising medication for older people.
Which interactions are most common and dangerous?
The combinations of most concern in older people are those that raise the risk of bleeding, damage the kidneys, cause drowsiness and falls or disturb potassium levels. The table summarises examples from official sources; it is not a complete list and does not replace a review by your pharmacist.
| Combination | Main risk | What the source says |
|---|---|---|
| Anticoagulant + anti-inflammatory (NSAID, such as ibuprofen) | Bleeding, especially in the gut | The STOPP/START 3 criteria flag it as a combination to avoid |
| ACE inhibitor or ARB + diuretic + NSAID (the "triple whammy") | Acute kidney failure | The Castilla-La Mancha bulletin describes it as common in older people |
| Benzodiazepine + opioid | Heavy sedation and breathing problems | The UK MHRA advises avoiding it unless there is no alternative |
| Two or more drugs with anticholinergic effects | Confusion, constipation, urinary retention, falls | STOPP/START 3 includes it as a specific criterion |
| Aldosterone antagonist + other drugs that raise potassium | Hyperkalaemia (high potassium) | STOPP/START 3 advises against combining them without monitoring potassium |
| St John's wort + warfarin, digoxin and others | Reduced effect of the medicine | The AEMPS has warned about this interaction |
The "triple whammy" deserves special mention because it is so common: many older people take a blood pressure tablet and a diuretic, and one day add ibuprofen for a sore knee. The Castilla-La Mancha bulletin notes that the risk is highest in the first 30 days of the combination and when the person is dehydrated. You can read more about these drugs in what NSAIDs are and how they work.
Which signs may point to an interaction?
Any new symptom that appears after starting, changing or adding a medicine could be an interaction. The MSD Manual advises professionals to assume that a new symptom in an older person may be drug related until proven otherwise.
- Drowsiness, confusion or disorientation that was not there before.
- Dizziness on standing up, unsteadiness or a fall.
- Bruising, bleeding gums, black stools or dark urine.
- Passing much less urine than usual, swelling or extreme tiredness.
- Palpitations, muscle weakness or cramps.
If you notice any of these signs, do not stop or change medicines on your own: call the health centre or ask your pharmacist, and go to A&E if there is heavy bleeding, difficulty breathing or severe drowsiness.
What can you do at home to prevent them?
The most effective step is to keep a single, up to date list of everything the person takes, including over the counter products and supplements, and to have it reviewed by the doctor and pharmacist. Always using the same pharmacy also helps to spot duplicates.
- Write a list with the name, dose and timing of each medicine, and take it to every appointment.
- Before buying anything without a prescription (an anti-inflammatory, a syrup, a herbal tea), ask at the pharmacy whether it is compatible.
- Ask the GP for a medication review at least once a year or after a hospital stay.
- Read the leaflet of every new medicine: the "other medicines" section warns about known interactions.
- Bear food in mind too: some foods interfere with certain treatments, as we explain in foods that interfere with medication.
A practical example: Antonio and the ibuprofen
Take the case of Antonio, 79, who takes an ACE inhibitor and a diuretic for his blood pressure and an anticoagulant for atrial fibrillation. During a hot week his knee hurts and his neighbour offers him ibuprofen. That combination brings together two interactions from the table: a higher risk of bleeding and a higher risk to the kidneys.
The sensible step would be for his daughter to call the pharmacy or the health centre before Antonio takes anything, give them the full list of medicines and ask which option is safe for the pain. If Antonio had already taken it and noticed he was passing little urine or had black stools, he should seek advice straight away.
How can IMA Health help?
At IMA Health we coordinate with a specialised pharmacy so that each person's medication is prepared by day and time of each dose (a personalised dosage system), with home delivery and personalised follow-up. Having all the medication centralised in one pharmacy makes it easier for the pharmacist to see the full treatment and spot duplicates or combinations worth checking with the doctor.
Sources
- Spanish version of the STOPP/START 3 criteria (Rev Esp Geriatr Gerontol, 2023)
- Castilla-La Mancha Pharmacotherapy Bulletin: kidney failure caused by the triple whammy
- AEMPS (Spanish Medicines Agency): Hypericum perforatum (St John's wort) and interactions
- MSD Manual: drug-related problems in older adults (Spanish edition)
- MHRA: benzodiazepines and opioids, risk of respiratory depression
This article is for information only and does not replace advice from your doctor or pharmacist.