All frogs are not poisonous, but some of them have a kind of venom, which they use when they are attacked by predators.
The common toad exudes poison through its skin if it is attacked. Cave toads contain a drug that can cause hallucinations if these are eaten.
The skin of some frogs and toads contains poisons, which are among the most powerful known to the humans.
Below Frogs info from Wikipedia (Read More):-
Many frogs contain mild toxins that make them unpalatable to potential predators. For example, all toads have large poison glands—the parotoid glands—located behind the eyes on the top of the head. Some frogs, such as some poison dart frogs, are especially toxic. The chemical makeup of toxins in frogs varies from irritants to hallucinogens, convulsants, nerve poisons, and vasoconstrictors. Many predators of frogs have adapted to tolerate high levels of these poisons. Others, including humans, may be severely affected.
Some frogs obtain poisons from the ants and other arthropods they eat;[8] others, such as the Australian Corroboree Frogs (Pseudophryne corroboree and Pseudophryne pengilleyi), can manufacture an alkaloid not derived from their diet.[9] Some native people of South America extract poison from the poison dart frogs and apply it to their darts for hunting,[10] although few species are toxic enough to be used for this purpose. It was previously a misconception the poison was placed on arrows rather than darts. The common name of these frogs was thus changed from "poison arrow frog" to "poison dart frog" in the early 1980s. Poisonous frogs tend to advertise their toxicity with bright colours, an adaptive strategy known as aposematism. There are at least two non-poisonous species of frogs in tropical America (Eleutherodactylus gaigei and Lithodytes lineatus) that mimic the colouration of dart poison frogs' coloration for self-protection (Batesian mimicry).
http://en.wikipedia.org/wiki/Frogs
