"Ingredients: aadvark, abdomen, aborigine..."
Each time it is consumed or cooked, it will pull and act like a random item of the table of *almost* all available cookable food items. This includes very dangerous ones such as Black Ooze. However, it does not include Neutron Flux, so there is no risk of insta-killing yourself.
When Cooking with this as a recipe via skill, it will create a randomized choices of effects but the recipe will be picked from the pool of one specific effect.
By using the Carbide Chef skill, you can create a persistent recipe using the random effect you got from a can, then reliably reproduce that effect in the future; in theory, this allows you to use Canned Have-It-All to replicate rare cooking effects, though in practice Canned Have-It-All itself is very rare and you would need to make recipes with it repeatedly until you got one you liked.
It is possible to use Canned Have-It-All to replicate the effects of Drop of Nectar, which, when eaten, has a 25% chance of adding +1 to all attributes. Combined with Carbide Chef, Precognition, Domination, and cloned Kippers (who sell this item), this can potentially be exploited indefinitely.