OBJECTIVES:   To study the extent of brand substitution and switching between Selective Serotonin  Reuptake Inhibitors (SSRI) available on the Pharmaceutical Benefit Scheme (PBS).   METHODS: PBS prescription claims data provided by Medicare Australia of a 10% random sample of all Australian long-term concession card holders covering the time period August  2007 through July 2008 were assessed.  Patients had to fill at least 4 prescriptions for an SSRI with generics over the one year period (fluoxetine [2 doses forms with up to 12 brands], fluvoxamine [2 strengths with up to 5 brands], paroxetine [1 strength with 11 brands] & sertraline [2 strengths with up to 13 brands]).  The proportions of non-switchers (single brand only) and multiple switchers (two or more switches between brands) were determined for each strength of each SSRI molecule.  RESULTS: A total of 18,691 Concessional patients filled at least 4 prescriptions for a SSRI in the 12 month time window. The majority of these patients received a single brand over the period: ranging from 49% for fluvoxamine to 67% for fluoxetine.  Only a small proportion received three or more brands: ranging from 12% for fluoxetine to 16% for fluvoxamine.    The proportions of multiple switchers varied slightly between molecules with: 22% for fluoxetine, 32% for fluvoxamine, 26% for paroxetine and 24% for sertraline.  Switching was greater for products where more brands were available.  The larger the brand premium, then the lower the market share of the premium brand.   More multiple switchers were aged under 50 years (31%) than 70-79 years (20%).  CONCLUSIONS:   Multiple switches between SSRI brands occur in around one in four Australian Concessional patients and this has the potential to confuse some patients if not adequately monitored by prescribers and pharmacists.  Switching rates increased where more brands are available on the PBS as well as in younger patients.