He-he, so I opened a book, and yes, Triss was allergic only to potions, the reason she could not heal herself with magic is that she could not concentrate because of convulsions and pain. She left her amulet in Kaer Morhain, and that's why she fell ill.
From Blood of Elves:
“I’m allergic!” she sobbed with helpless exasperation and despairing anger. “I always have been! I can’t tolerate elixirs! I can treat others with them but can only treat myself with amulets.”
“Where is the amulet?”
“I don’t know.” She ground her teeth. “I must have left it in Kaer Morhen. Or lost it—”
“Damn it. What are we going to do? Maybe you should cast a spell on yourself?”
“I’ve tried. And this is the result. I can’t concentrate because of this cramp…”
Thank you. But i thought magic and elixir is different ?
Magic is something mage can use everywhere, usually from their hand. While elixir is like potion mage just bring with them