An assembled blister package has a blister sheet formed with blisters sealed by a foil backing sheet. The blister package is sandwiched between opposite sides of a folded card (not shown) to stiffen it. Each of these sides has a rectilinear array of windows defining discrete areas on which to print. A header sheet on which a bar-code is printed is attached to the backing sheet, and a reader is positioned to read the bar-code and extract from it information which is fed to an ink-jet printer. Two stepping motors are used to control the position of the printer as information is printed on the respective discrete areas of the array. The printer can be spaced from the backing sheet by a sufficient distance to enable it to print without obstruction.