OBJECTIVES: Seniors are estimated to account for about 40% of all Canadian retail spending on prescription drugs, and make up a significant proportion of public drug program expenditures.  This analysis examines the types of drugs accounting for the majority of public drug program spending for seniors, and the distribution of program spending across seniors. METHODS: This study examined claims for 1,039,642 seniors on public drug programs in six Canadian provinces (Alberta, Saskatchewan, Manitoba, New Brunswick, Nova Scotia and Prince Edward Island) from 2002 to 2008, representing over 80% of the senior population in those provinces in 2008. Public drug program spending on seniors refers only to the portion of prescription costs paid by the public drug program, including professional fees and markup, where applicable. RESULTS: In 2008, the top 10 drug classes, in terms of public drug program spending, accounted for almost half (48.3%) of all spending on seniors in the six provinces. 3-hydroxy-3-methyl-glutaryl-Coenzyme A reductase inhibitors accounted for the highest proportion of drug program spending for seniors, while 4 of the top 10 classes were used to treat hypertension. Almost half (45.5%) of public drug program spending was for a small group of seniors (14.7%), where the drug programs paid $2,500 or more of their annual drug costs. Tumour necrosis factor alpha inhibitors were the fastest growing drug class, in terms of spending, growing at an average annual rate of 58.4% between 2002 and 2008. CONCLUSIONS: Findings show almost half of drug program spending for seniors is on a small number of drugs.  This suggests that any cost saving initiatives that impact spending on these drugs could have a significant impact on overall public drug program spending.