Platypus venom can have you in excruciating pain for months! Even morphine doesn't work against it!

It isn’t lethal but can cause excruciating pain for months and morphine isn’t effective! The platypus is one of only a few animals that is able to produce venom, and although the venom is nonlethal, it is excruciatingly painful and is extremely difficult to treat because morphine is ineffective. 

The venom is made in the crural glands of the male platypus, located in the upper thigh. The gland produces venom that contains around 19 different peptides and primarily causes lower blood pressure and immense pain as blood is redirected toward the wound. 

And while it consists of similar substances as reptile venom, there is no necrotic component and only muscle wastage is the result in humans. However, once bitten pain can last up to several months and the pain would not respond to morphine, meaning if you get bit by a platypus you basically have to suffer for a month until it stops on its own! 

(Source)