Cart hesitates for a moment, but then accepts the offered potion. He chugs it down, grimacing at the taste. You wait...
Minutes crawl by as you carefully observe your companion, the store owner lights a pipe and lounges against the counter, blowing smoke rings.
"I don't think it's working. Maybe- arghhh!" Cart doubles over, the flask clattering against the wooden floorboards. The wolfmorph rolls on to his back, hands clawing desperately at his side. You feel the room's temperature rise and the air become thick with demonic magic. A cracking fills the air as Cart's bones begin to reknit themselves. The bruises fade from his skin and his misshapen ribcage begins to realign.
Is it just your imagination, or is steam rolling off of Cart's body? You feel like if you flicked a drop of water at his skin, it would sizzle with heat. A few moments after Cart's wounds heal, it becomes apparent that the potion's magic is not done. Having made its host whole once more, the dark brew begins to work other changes. The sinewy muscles of Cart's forearm's swell with vigor and you hear the tearing of linen as his clothes strain to contain an ever strengthening musculature. The wolfmorph's fangs and claws take on fearsome proportions and his fur turns shaggy and black as midnight. Your companion convulses, knocking over furniture and tearing deep rends in the floor.
Then just as suddenly as it started, the transformation ends. Cart slumps with exhaustion, breathing heavily. The storeowner approaches.
"Well that went better than expected. You must be one tough puppy to be done with the full dosage so soon. Now let's check those rib-"
She let's out a strangled yelp as Cart surges to his feet. He wraps the busty half-elf in a fearsome hold, pawing at her chest and snarling in her ear. The straining at his breaches lets you know that Cart's other parts have gone through a growth spurt. His eyes glow like a pair of coals with a fearsome rage.
"L-lust madness... N-needs... Catalytic release..."
Seems like Cart's sanity is in danger of being burnt away by the demon brew. Preserving your companion's personality will likely take some finesse... And a lot of spellpower. Then again, you could let the situation take its course and gain a powerful, albeit corrupt minion.