Abstract: A blister pack arrangement comprises at least one blister pack with individually openable blisters including a number of blisters containing placebo (placebo blisters), and a number of blisters containing an active pharmaceutical drug (drug blisters). Indicia means provides user information that indicates a predetermined sequence in which said blisters are to be opened during a trial period, said sequence being such that the placebo blisters are to be opened before the drug blisters. The blisters are not identifiable with each other either by sight or by said indicia means. An initially secret treatment code is to be broken after the trial period, revealing when in said sequence a switch is made from placebo to active drug.