Some nasty chemicals are used to treat timber, but they're designed to not leach out of the wood into any surrounding soil, so chemicals affecting the plant shouldn't be an issue (from what I've read/understand).
I use old (untreated) pallets to make grow boxes - they won't last forever (maybe a few years if I'm lucky) but the price is right, so that's an option to consider if you're concerned about using treated wood.