system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is is that a venomous animal can bite you anywhere on your body and it be a danger, but in medicine they have to find a vein to inject drugs into you?
gpt
Most forms of venom destroy tissue and cells by attacking and digesting their walls. You actually lose the flesh where the animal bites you. If it's injected into the bloodstream, depending on its quantity and strength, it may destroy enough blood and parts of the circulatory system to kill you quickly; if it's injected into a more localized part of the body such as a hand, that part might be seriously damaged but if enough of it doesn't enter the bloodstream and get carried around it won't move to and damage other parts. You could still easily die of gangrene or other secondary complications however.

Needle-delivered medicine is typically intended for either the whole body or a large part of it. In many cases, such as a drip-delivered nutrient and saline solution, the easiest and fastest way to distribute it through the entire body is the bloodstream. Miss the vein and it just sits in the local tissue and doesn't reach where it's supposed to go.