Not to bump an old thread but I thought I'd contribute.
I'm a chronic pain patient and get a shit ton of Soma RX'd along with Oxycodone. Both my Dr and my pharmacist state there is no real danger to mixing the two other than passing out. They both agree that it's nowhere NEAR as dangerous as mixing a benzo and an opiate of any kind. I'm assuming there's some kind of biochemistry related reason, even though they're both CNS depressants.
Anyway, I'm tolerant to about 60mg of Oxy at one time, and I just took 50mg of Oxy and 1 Soma and I feel VERY good, but nowhere near any kind of OD. I'm still well awake and my breathing is more or less normal. Adding in one or two more Soma most likely wouldn't be an issue.
I think caution is always the name of the game, but Soma and opiates seems to be relatively safe, even if the Carisoprodal does indeed potentiate opiates. It doesn't seem to trigger the majorly dangerous CNS depression that benzos and opiates can.
I will say that Soma is defintely an unpredictable drug. Sometimes I'll take my dose and not feel much, other times I'll take the same dose and be fucked up. It's weird, but there's no consistency to the effects. Sometimes it hits and others it doesn't, as someone else mentioned. I'd imagine taking a few Soma and having them all 'hit' + a large opiate dose would be a bit more dangerous, of course.