mostly because I was writing a shifter romance and wanted to get my lore straight. In a lot of modern Western pop culture, they're basically synonyms, but if you go back further, the vibe is different. A werewolf is typically a human cursed or infected, forced to change by the full moon, and it's a monstrous, painful thing. Think 'An American Werewolf in London' – tragic, bloody, a disease. The lycanthrope, from the Greek 'lykos,' feels more like a voluntary shapeshifter, or at least someone with control. It's more of a permanent state, a dual-nature being.
That said, cross-culturally it gets wild. The Norse had the 'berserkers' who wore wolf pelts and went into a battle frenzy, which is more of a spiritual possession. In Japanese folklore, you've got the 'werewolf' idea popping up via Western influence, but their native 'wolf-god' or 'ōkami' figures are more like deities or guardians of the mountains, not cursed humans at all.
Honestly, I think the romance genre, especially Omegaverse and pack stories, has mashed the two together into something new entirely. The 'lycan' is often a noble, ancient species with complex social hierarchies, while 'werewolf' gets used for the rougher, more bestial side of the same coin. It's less about cultural accuracy now and more about what flavor of supernatural tension you want in your story.