safety in early pregnancy is mixed as of 2017. The American College of Obstetricians and Gynecologists states that while they can be used in the first trimester other options may be preferred. They remain a first line treatment in the second trimester. A 2015 meta analysis found no increased risk from first trimester use in cohort studies that was a slight increase of malformations in case control studies. Available forms There are two formulations of nitrofurantoin: Macrocrystals - (Macrodantin, Furadantin) – 25, 50, or 100mg capsules – taken once every 6 hours Monohydrate/macrocrystals - (Macrobid) – 100mg capsules – taken once every 12 hours or 2 times a day (written on prescriptions as BID, which is the last part of the trade name MacroBID). It is 75% monohydrate and 25% macrocrystals. Contraindications Nitrofurantoin is contraindicated in patients with decreased renal function (CrCl < 60 ml/min) due to systemic accumulation and subtherapeutic levels reached in the urinary tract. However, a retrospective chart review suggests the data for this cutoff are slim and a cutoff of CrCl < 40 ml/min would be more appropriate. Many of the severe side effects of this drug are more common in the elderly and those with renal impairment, as this causes the drug to be retained in the body and reach higher systemic levels. Thus, the drug is not recommended for the elderly population according to 2012 AGS Beers criteria. Nitrofurantoin is also contraindicated in babies up to the age of one month, as they have immature enzyme systems in their red blood cells (glutathione instability), so nitrofurantoin must not be used because it can cause haemolytic anaemia. For the same reason, nitrofurantoin should not be given to pregnant women after 38 weeks of pregnancy. Nitrofurantoin is contraindicated in patients with glucose-6-phosphate dehydrogenase deficiency (G6PD) because of risk of intravascular hemolysis resulting in anemia. Adverse effects The most common side effects with nitrofurantoin are nausea, headache, and flatulence. Less common adverse events (occurring in less than 1% of those taking the drug) include: Gastrointestinal: diarrhea, dyspepsia, abdominal pain, constipation, emesis Neurologic: dizziness, drowsiness, amblyopia Respiratory: acute pulmonary hypersensitivity reaction Allergic: pruritus, urticaria Dermatologic: hair loss Miscellaneous: fever, chills, malaise Patients should be informed that nitrofurantoin colors urine brown; this is completely harmless. Some of the more serious but rare side effects of nitrofurantoin have been a cause of concern. These include pulmonary reactions, hepatotoxicity, and neuropathy.