Is rust in my tap water a poison?

No: No but it is a good source of iron! are your anemic? I would have my water checked, consider havign a plumber check things out and consider some sort of filtration or water softening. You could have some really bad plumbing or a bad water heater. Check it out!

No: Rust is oxidized iron. It can originate anywhere from a water main to your own plumbing. It can come from iron sediment inside corroded iron or steel pipes or can occur naturally in some drinking water sources, Though rusty water may look and taste unpleasant it is not a health concern (with the possible exception for people with hemochromatosis a rare disorder of excess body iron).