Bear in mind that moisture in the raised bed will travel outwards to
evaporate on the dryer side so it is very unlikely that anything in solution
would be taken up by plants in the bed. If there was the possibility of
dangerous chemicals being leached from such timber, I'm sure the EU would
have banned the use of it! If that timber is what is termed "gravel board"
for fencing, I've been using it for years and apart from growing another
head . . . . !!!!!

Geoff