The one time I mixed em I had a golf ball sized lump in my left glute for 5 days, but it went away just fine. Could have been a bad pin, could have just missed the muscle (used a 1.5" pin) but that would be the worst pin I've had in two years of pinning. I will not mix them again. Both vials I had used before and after the bad pin to no ill effect. So many say it's fine though. So I'd say try it to find out.