Purpose To show the necessity of distinguishing several patterns of drug prescribing that may lead to co‐medication. It is demonstrated how these different patterns can be investigated using large databases containing pharmacy data or reimbursement data. Methods Two examples illustrate how the particular pattern of co‐medication studied will influence the reported proportion of patients having co‐medication, the use of antidepressants among people using anticonvulsants, and the use of antihistamines among people receiving penicillines. Results Depending on definition and period considered, the percentage of anticonvulsant users co‐medicated with antidepressants ranged from 5.8% (95%CI 5.0%, 6.8%) to 14.5% (95%CI 13.2%, 15.9%) in 2000. Comparing 2002 with 2000, the ratio of proportions ranged from 1.3 to 2.1. The percentage of people who received penicillines and were co‐medicated with antihistamines ranged from 0.5% (95%CI 0.4%, 0.6%) to 9.7% (95%CI 9.3%, 10.2%) in 2000. Comparing 2002 with 2000, the ratio of proportions ranged from 1.2 to 1.6. Conclusion The co‐medication patterns investigated yielded clinical as well as statistically significant different estimates. The estimates differed up to a factor 2.5 for the drugs usually prescribed for long periods, and a factor 12 for drugs prescribed for short periods. Hence, we propose to distinguish the patterns ‘co‐prescribing’, ‘concomitant medication,’ and ‘possibly concurrent medication.’ The research question determines the co‐medication pattern of interest, and the drug and disease under study determine the time window. Copyright © 2006 John Wiley & Sons, Ltd.