It doesn't work and a collector told me that if someone takes pyridium and then gives a sample that has pyridium in it (which is obvious to them) then they reject the sample and you have to give another sample without pyridium in it. When a collector rejects a sample sometimes there are guidelines (depending on why they are failing the sample) they have to follow like getting a second sample within 3 hours and you are allowed so many ounces of water, but with pyridium it lasts about 8 hours so your second sample would probably be rejected too. Depending on who the test is for, a bad sample can be considered refusal to donate and that can result in an automatic failure.