BACKGROUND: Although cholinesterase inhibitors are used for the treatment of Alzheimer's disease, the clinical benefits of these drugs are being questioned. Anticholinergic drugs have the opposite pharmacological action to cholinesterase inhibitors, and may antagonize the effects of cholinesterase inhibitors. Therefore, this drug combination should be avoided. Nevertheless, high rates of concurrent use of anticholinergic drugs and cholinesterase inhibitors have been reported in the US. To the authors' knowledge, use of this inappropriate drug combination has not been studied outside North America. OBJECTIVE: To investigate (i) whether anticholinergic drug use is more common among users than non-users of cholinesterase inhibitors, and (ii) which factors are associated with use of anticholinergic drugs among users of cholinesterase inhibitors. METHODS: We analysed data on age, sex, type of residential area (urban/rural) and drugs dispensed for patients aged>or=75 years registered in the Swedish Prescribed Drug Register from October to December 2005 (n=731,105). The prevalence of use of anticholinergic drugs in users of cholinesterase inhibitors was compared with that in non-users of cholinesterase inhibitors and logistic regression was used to study the association between use of cholinesterase inhibitors and anticholinergic drugs. Logistic regression was also used to analyse whether age, sex, type of residential area or number of dispensed drugs was associated with use of anticholinergic drugs among users of cholinesterase inhibitors (n=18,326). RESULTS: Anticholinergic drug use was more common among cholinesterase inhibitor users than non-users, particularly in men, of whom 9% who were taking cholinesterase inhibitors were dispensed anticholinergic drugs compared with 5% who were not taking cholinesterase inhibitors. Use of cholinesterase inhibitors was associated with use of anticholinergic drugs in men (odds ratio 1.23; 95% CI 1.13, 1.35), after adjustment for age, type of residential area and number of dispensed drugs. Male sex and use of many drugs were independently associated with concurrent use of anticholinergic drugs and cholinesterase inhibitors. CONCLUSION: Anticholinergic drug use is more common among cholinesterase inhibitor users than non-users, even though anticholinergic drugs may antagonize the effect of cholinesterase inhibitors. Hence, if the true clinical effects of cholinesterase inhibitors are to be accurately assessed, they need to be studied in the absence of anticholinergic drugs.