As the title says, poison is to poisoned as venom is to what?
I tried looking up venomed but it means something different.
Is there such a word?
In biology, the term envenomated (past participle of envenomate) is used for this; Google Books turns up uses like:
That said, I think I'd only use envenomated if the venom is actually delivered by a venomous animal (especially a snake, via bite). If, say, someone were to extract snake venom and use it to poison someone's drink, I think the recipient would be poisoned, not "envenomated".
To envenom someone or something is to make it poisonous or to add poison to it. So, if using poison in the figurative sense of fouling, embittering, spoiling, etc. it would be acceptable, as in envenoming a relationship. It would not be suitable when using poison in the sense of murdering someone by poison, or adulterating something lethally.
The way I see it is that 'venom' and 'poison' are both nouns. They are also both verbs; however, in reality, we don't actually hear 'vemon' used as a verb. Some dictionary entries depict 'venom' only as a noun (http://www.thefreedictionary.com/venom) whereas those that depict 'venom' also as a verb tend to state that its usage as a verb is archaic (http://dictionary.reference.com/browse/venom).
What I am getting at is that when we say 'poison is to poisoned' we are talking about 'poison' as a verb. Technically you could say 'as venom is to venomed' (http://verbs.woxikon.com/english-verbforms/venom.php); however, since 'venom' is only ever used as a noun, we are attempting to compare a verb ('poison') and a noun ('venom') in a manner that is incomparable. I cannot think of one native English speaker I know who would have ever heard the word 'venomed'. So whilst it may be grammatically correct, I don't think that you would draw this comparison if your comparison is related to usage as as opposed to grammatical technicalities (Thus, I look to usage to answer your question not to the few authorities that stipulate that 'venom' is an archaic verb.)
Some responses have mentioned the adjectives 'poisonous' and 'venomous'. We could indeed say that, 'poison is to poisonous as venom is to venomous' (noun --> adjective, noun --> adjective) since they are both nouns in the first instance (as opposed to 'poison' as a verb and 'venom' as a noun which is my interpretation of the phrase in question).