I will tell you so honestly, until we got Toby's drugs worked out for his SM, his was very aggressive...would growl at my face if I even tried to kiss him. It would make me so upset. But I didn't know why then.
We adopted Toby when he was 5 1/2, he showed aggression right away...to dogs and people. I wondered if we could even go on. Then I got hi MRI'ed (due to other factors) and found out he had SM. We got him on Gabapentin and even tried anxiety drugs like prozac, and he did okay for a month or two, but still seemed not okay when we got around his head 70% of the time and hated dogs even getting near. Then he started showing other signs of pain and we went back to the neuro knowing he was still not okay - so we switched to Lyrica and Omeprazole and...his whole world changed....so did ours. It has been a month and a half...no growling at me, no aggressive episodes, nothing. He can even walk past dogs (still is not a dog dog, but less reactive) and just so much more relaxed and okay.
I am not saying to switch meds, that is a neuro call and every dog is different, but I can say that SM, in my experience, does cause behaviour changes....and it's not their fault.
Having had a surgery gone wrong a few years ago and being in chronic pain myself (at a very young age), I can attest that pain alters behaviour and emotion. My hubby and I always talk about it - but dogs can't, and when they feel like crap, their only mode of "it hurts!" And to protect is to act.
Just sharing my story...SM has been one of my hardest journeys...but also, taught me a world about life, dogs, care and selflessness.