The panels can be run horizontally but the manufacturers do not recommend this method as both the tongue and groove sides of the joint design have recesses that could hold water. In living rooms or outside of the bathroom or shower room this is not so much of an issue but obviously is not ideal in wet areas.
Fitting the panels horizontally also makes installation a lot more difficult as you have to maintain a horizontal line around the room, even if the walls are out-of-true, and you don't have a variable grout gap as you would do with tiles to help compensate for any discrepancies.
