The formal definition of "ditto" doesn't quite mean "likewise," true, but at least in American English it's a widely accepted slang usage. I wouldn't use it in formal language, but I also wouldn't call it incorrect per se.
"Idem," it seems to me, is a much less general term, not to mention a very uncommon one. Wiktionary does list "idem" as a synonym of "ditto" and "likewise," but I think a better synonym is "ibid." According to the New Oxford American Dictionary, "idem" is "used in citations to indicate an author or work that has just been mentioned: Marianne Elliott, Partners in Revolution, 1982; idem, Wolfe Tone, 1989." Merriam Webster defines "idem" as "the same as something previously mentioned — used chiefly in bibliographies."
Maybe try "same," as in:
"I am completely addicted to chocolate."
This isn't really any less informal than "ditto," but you might prefer it. I'll add as a caveat that I don't know how regional this usage of "same" is or isn't. (I'm from the San Francisco Bay Area.)