Nitrofurantoin
Nitrofurantoin is an antibacterial that concentrates in the urine rather than in the bloodstream. Bacteria reduce it into reactive compounds that damage their DNA, ribosomes and enzymes at several points at once. It is used for bacterial infections of the bladder and lower urinary tract, and is not suitable for infections elsewhere in the body.
It is taken with food, which improves absorption and reduces stomach upset. The urine commonly turns dark yellow or brown, which is expected and not a sign of harm. Because the drug has to be filtered into the urine to work, reduced kidney function both weakens it and lets it accumulate, so creatinine and eGFR are checked when kidney disease is present; ALT may also be followed, as the liver is occasionally affected. The full course is completed even once symptoms settle, or the infection can return.
See a doctor for new shortness of breath, a dry cough, chest pain or fever, especially during long-term use, as lung reactions are a recognised though rare problem. Numbness or tingling in the limbs, yellowing of the skin or eyes, or symptoms that do not improve also need review.
Reference information, not medical advice. Discuss any change with your doctor.