The majority of patients receiving palliative care are likely to be elderly and to be taking several different drugs both regularly and p.r.n. to alleviate their symptoms and for pre-existing chronic conditions. The aim of this survey was to identify and quantify the drug combinations that result in clinically important interactions involving cytochrome P450 (CYP) in patients attending specialist palliative day care centres.

Day patients attending one of four specialist palliative care units during 1 week in September 2003 were surveyed. For each patient, the likelihood of a CYP-mediated interaction between individual drug combinations was assessed based on established databases (Web of Science, PubMed, http:/www.gentest.com/human_p450_database/index.html), Stockley’s ‘Drug Interactions’ (The Pharmaceutical Press, 2002), and personal files of one of the authors (MSL).

One hundred and sixty patients (87 (54%) males) with a mean (SD, range) age of 69 (13, 25-93) years, and all but 8 (5%) with cancer (motor neurone disease 5, multiple sclerosis 3) were surveyed. Patients took a mean (median, range) of 6.3 (6, 0–16) and 1.5 (1, 0–6) regular and p.r.n. drugs, respectively, giving a combined total of 7.4 (7, 1–17) different drugs. The majority received one (22 (14%) or two or more (123 (77%)) that were substrates, inhibitors or inducers of one of the major human CYPs with a mean (median, range) of 3.7 (4, 0–12) drugs. Two hundred and thirty-three, 146, 137, 63 and 12 prescriptions were written for drugs interacting with CYP3A, CYP2C19, CYP2D6, CYP2C9 and CYP1A2, respectively. Twenty-five drug combinations were categorized as giving rise to clinically important or potentially clinically important drug interactions, affecting 35 (22%) patients. These were: ‘important’ omeprazole/diazepam, phenytoin/dexamethasone; ‘potentially important’ dexamethasone combined with temazepam or amitriptyline or fentanyl or quinine or simvastatin or tacrolimus or zopiclone, prednisolone combined with diazepam or amlodipine or fentanyl or trazodone or zopiclone, levopromazine combined with oxycodone or haloperidol or tamoxifen, haloperidol/oxycodone, dextropropoxyphene/tramadol, carbamazepine/zopiclone, coproxamol/codeine, fluoxetine/codeine, haloperidol/codeine, verapamil/zopiclone.

The finding that almost a quarter of the patients surveyed were receiving combinations of drugs that give rise to clinically important or potentially clinically important interactions involving CYP, emphasises the importance of routinely considering the risk of an interaction when making prescribing decisions in palliative care.