.. and to add to David's good posting -
Pyridium will most likely darken the color of your urine to an orange or red color. This is a normal effect and is not cause for alarm unless you have other symptoms such as pale or yellowed skin, fever, stomach pain, nausea, and vomiting. Darkened urine may also cause stains to your underwear, which may or may not be removed by laundering.
-all the best