By definition, venomous (in your sense of the word) describes an animal capable if injecting a poison (although my experience with stinging nettles may cause me to define it differently). Some venomous animals include spiders, bees, scorpions, jellyfish, some spiny fish, and the blue-ringed octopus.
venomous: (of an animal) having a gland or glands for secreting venom; able to inflict a poisonous bite or sting.
Therefore, venom (the secretory product) cannot be venomous.
Poison, however, is poisonous. Poisonous describes its potential effect on the body. Some poisonous animals include puffer fish, monarch butterflies, birds who feed on poisonous insects, etc. Poisonous plants make poison, but unless it is ingested (has an effect on the body), it does not act as a poison. If it is a skin irritant, I'd say it was a topical irritant. (Maybe too specific for you.)
Poisonous: (Medicine) having the effects or qualities of a poison; (of a substance or plant) causing or capable of causing death or illness if taken into the body (usually by ingestion or inhalation).
Informally, poisonous snake is synonymous with venomous snake. However, not all poisonous animals are venomous (poison dart frogs secrete a toxin which is one of the strongest poisons produced by an animal.) It is sometimes called a venom, but doesn't fit the usual definition of venom. It becomes a venom when used on dart-tips.
Edited to clarify and add: some venoms are poisonous if swallowed, but most are not. Snake venom is usually harmless if swallowed, but poison dart frogs are poisonous if swallowed.
I would say, vemon can be poisonous, but venom can't be venomous.
