Every thing I have read says it is safe, but who knows. Wide boards may not be available. Cedar certainly looks nice. Both are expensive. I use construction grade lumber,
it lasts 10 years or so until the fungi & ants have their way. If you are reasonably sure that you wont change the beds in the future take a look at the forum below. Some far out stuff, all about making concrete look amazingly natural

Here is a link that might be useful: hypertufa forum

Trex, a building material for decks and such, is an alternative to pressure treated lumber that may be less acceptable for garden work. Nothing to date indicates that this recycled plastic material poses a problem in the garden, but then it took almost 50 years to discover that CCA PT lumber posed an arsenic hazard.

And because it is plastic based, you do (well, maybe not DO, maybe should) also have to consider the hidden costs of the plastic - I don't think it's recycled plastic bottles. If it is, the hidden costs go down a bit. And, while it is wood chips/fibers, if it is virgin wood, there are hidden costs there as well - trees cut for the chips, branches that would remanin on the ground to rot being chipped, etc.. It DOES/can use smaller or "trash" trees, but that means more of a clear-cut than not. It also means less wood going into land fills, and it seems to have a long life span, so will need to be replaced less often, on the plus side. I don't think plastic as used in Trex out-gasses, but I may be wrong - plasticizers in other plastics do, and they are not good chemicals for people, and probably not the environment. Along with the planks being long-lasting is the fact that it will probably NEVER rot - I have picked up plastic bags that I KNOW are 15 years old which looked as good as new. I'm NOT saying flatly don't use it, just adding a few more things for you to consider. I know, it's just what you needed.....

If you think you may only use a raised bed for 4-6,7 years you might try rough cut hemlock, if it is local to you. I have them in my greenhouse and they are holding up fine. I use 2 Xs, as the 1 Xs rot by about the 3rd year.
Regarding another posters statement about clear-cutting, it all depends on where you are located. In my area of PA, when a logging job is done (not clear cut) there is always lower grade wood that must be removed even if there isn't a use for it.
As for looks, I like the natural look of the rough cut, but if you want a fancier look, probably it is not for you. Rough cut is a lot cheaper than construction grade lumber also.

Trex is made from reclaimed and recycled material such as wood fibres, plastic grocery bags, and numerous other similar things.

Kimmsr, thanks for the info. Now that you mention it, yes, I did know that Trex was recycled plastics, but couldn't remember at the time.

It's not that I am against Trex and similar products, I'm just saying be aware of ramifications and make educated decisions. In the long run, the manufactured products may be better for the environment than using lumber since you can harvest trees at a younger age, using a more-rapid turnover for planted forests, which SHOULD take some of the pressure off old-growth forests. As well, the recycling of the plastics means less of them get to the landfill. I just don't LIKE plastics.

however, I may, in fact, use Trex for the footing and walkway of a covered walkway between my new garage and the house, since some of the drainage from that side of the land will run through there, practically guaranteeing rot in rapid order for any wood used.

Anniew, it sounds like we're close-ish. Are you in NEPA?

We have lumber mill right here in town. Most of what they cut there is from local backyard logging. I've made my raised beds from plain old untreated construction grade 2x6's, and 5 years later only two boards are rotting, so I'm replacing this spring. It sounds like the rough cut would be cheaper, more rustic and it's easy for me to get very locally. Does bark rot more slowly?

Trex sells a few different kinds of material.

All the NON PVC products will eventually rot or decompose.

Same with TimberTech.

Azek and LifeTime Lumber are your best bet.

If you live anywhere near a saw mill they often have something called, "OUT LUMBER", It is rough sawn hardwood that is not suited for the furniture manufactures. This has knots or staining. You can usually buy this for near nothing. The length and width may vary, but the thickness is usually consistant. 4/4, 5/4, 6/4 is the usual thickness