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Forces holder to move last in any given turn. If Snorlax breeds while holding Full Incense, a Munchlax Egg will be produced as opposed to a Snorlax Egg.
Well, if you really want to move last every turn, then Full Incense will serve you well. For those of us with more than two brain cells, however, just about any other item in the game would be better. Certainly, Full Incense can be Tricked onto a fast opponent to cripple it, but even then, there are better ways of crippling an opponent that don't leave you automatically at a disadvantage, such as Choice items. Aside from that, it has no real uses competitively.
Driftveil Market (Post-National Pokédex) - 9,600 Poké Dollars.