I was an inexperienced cleric in the old times… Then I caught lycanthrophy.
Now that I'm a werewolf and humans are food to me, can I use the cantrip Purify Food and Drink to purify humans?
Sure, once they're dead and just another kind of meat. Casting purify food and drink on live creatures doesn't work for anyone, regardless of what the caster eats:
the spell has no effect on creatures of any type
This works on pretty much anything, alive or dead. Being a werewolf is not necessary. The spell targets "1 cu. ft./level of contaminated food and water" which means as long as your target contains some water or you supply some you can target anything that is 'food'.
A classic, but terrible, GM solution to players using this on edible substances (like humans) is to say that is only functions if the target is food to you (i.e. you eat it regularly/have eaten it before/etc). This is not how spells normally work in 3.5 and also doesn't make very much sense (i.e. the interaction of this spell with things like Spell Turning suddenly becomes unclear).
It also isn't a reasonable definition of food; when I talk about the Rock-Eating Monster I may well say "That granite sure looks like it'd be good Rock-Eating Monster food!" and this use of 'food' seems legitimate for descriptively and prescriptively. To understand what we can target with the spell, we need to figure out what food means. There are a couple of ways of defining it that seem reasonable:
1) Something is food if it is food to anything. If it can be shown to be rightly classified as food, which it probably can be if anything draws nourishment from it by consuming it, than it must be food and the ontology of 'food' as a classification does not permit that something might be food at one moment but then not food at another without anything about the thing changing. This has the advantage of being simple.
2) Something is food if it is food right now. Words are not properly understood to refer to any real objective truth, but only as how we use them. A human is food only if the word is used properly in describing the human. A different description, used at the same time, might not properly call the human food. The 'meaning' of words is their 'use. So you cannot purify even a banana except as part of its use as food, and the same would apply to humans. Recommended reading: Philosophical Investigations. This has the advantage of being a semi-reasonable approach to understanding the ontology of words. It has the disadvantage that the system is likely to break down in play, unless the GM (at least, and it's better if the players do as well) has a good understanding of Wittgenstein's theory of language. Also it's nearly equivalent to the first option, just with nuances that remove the incorrect statements about ontology in the first option.
Both of these allow you to, in at least some situations, target parts of a human as food.
Seems like lycanthropy is kind of a temporary thing- even if you're a werewolf, aren't you just a wolf at certain times? While you're a wolf, humans are food, and I'd say the spell works. While you're a biped, they really aren't food, so it doesn't.