Are printed circuit boards poisonous?

The short answer is no, but be careful anyway.

The board itself is made of paper and epoxy resin which is not toxic but is not good for you. The real problem is the components on the board and the solder. All solder on older boards will contain lead, but most newer boards use lead free solder as much as possible. Electrolytic capacitors if they burst can release corrosive chemicals and many other components contain mercury, gold, silver and various heavy metals. In other words don't eat it and be sure to wash your hands after handling before you eat or smoke.

There are often flame retardants that are hazardous (carcinogenic) in PC boards.