IME, cannabis can be a great source of relief in both opiate and benzo withdrawal - if you like cannabis in the first place. BUT, it depends on the strain: old-fashioned, non-hydro herb like Thai was wonderful, coming off of pharm opiates, and good, CBD-filled, calming hash was better still. Sadly, they only showed up now and again - there's so much damn skunk and hydro around that hash (I'm in Europe, it used to be more common than weed) has become harder to find. The high-grade skunk and name-brand strains of herb aren't as useful - they can increase anxiety drastically and even trigger panic attacks in opiate and benzo W/D. Also, they're tough on the wallet.
As to synth 'noids in blends: YMMV, but I found the JWH-blends (the better ones, anyway), a decent substitute for schwag or even mids. Those got banned here, and moving to a city where I can get anything BUT cannabis, I've occasionally succumbed to temptation and used AM-series blends while tapering off of benzos and chipping painkillers (I used daily for about 8 months, but have been able to have the odd indulgence since kicking without falling back into opiate addiction...yet. But as I type this I'm thinking it's time to have one last indulgence tomorrow, when I have the day off and the house to myself...then quit for at least a few weeks). Although the AM blends are skunky and slap your third neural circuit like a special delivery from William S. B in Sheol, they've been more useful,for me, than high-THC strains of hydro in alleviating benzo w/d anxiety. Problem is, as noted by other posters, the high's very short-lived - you're pretty much back to baseline after half an hour. Still, used judiciously, I find a few joints after my 'scripted nightime taper dose can help with sleep. More often, though, I buy three grams and chainsmoke till it's gone, with tolerance rapidly diminishing its efficacy and a mild rebound depression the next day. It's the junk food of cannabis substitutes - a pipe of good hash would be the fillet mignon of w/d assistance, with perfect fries and a side-order of well-seasoned spinach. The AM-blend's are more like a whopper, at best: some of them as unsatisfying and sickly as a Big Mac. Damn, I miss the US...and Wendy's.
And Lortabs.
Irony of ironies: I'm popping massive doses of DHC and morphine once or twice a week, just for a little comfort round the rough edges of the taper, and I wouldn't be doing it - or at least, nowhere near as often - if I had access to cheap Thai weed or good North African hash (Pollen or blonde especially). But in my current location, I'm short on contacts, pharms/H/rock/X/skunk are all that's on offer, and no-one can hook me up for some straightforward, old school herb or solid. Pricey skunk wouldn't be too hard to find, but isn't what I need at all. It's like craving a beer when all you can get is bathtub absinthe. And the hydro shit costs so much that it's shocking. I'd rather have a dime bag of NYC or SF schwag than an 8th of skunk-headfuck, most of all in benzo withdrawal. Ah, for the old days, before skunk bumped up prices and left half my generation paranoid. But then, THEY ARE out to get us...
Still, skunk with a short course of benzos to keep things calm can do the job at a pinch, making physical opiate w/ds much easier...but the operative words here are 'short course'. I wish I'd tapered off the benzos before kicking the morphine/fent/oxy et cetera: they somehow didn't sink their teeth into me that deep. The benzos, however, bit hard and are a long taper away from letting go. Oh, for just one of the ten-gram blocks of Moroccan squidgy black I used to get for half the price of a quarter of name brand skunk...t'ain't fair.
