There are many more chemicals in the world than there are letters in the alphabet or syllables in the English language. Certainly many more chemicals than the syllables Hazel knew. She was two weeks too late to realize "oxytocin" and "oxycontin" weren't the same thing.
Oxytocin was a chemical related to love, and particularly the love that formed in a mother towards her child. Too little of it might have explained why Hazel's mother never wanted to talk, never made her lunch, and was so out of patience with her at every question. If there wasn't enough and her mother didn't want her, though, oxycontin didn't fix anything. It only created an addiction that took her further from Hazel.
She hated chemistry ever afterward.