Antibacterial activity
Nitrofurantoin has been shown to have good activity against:

E. coli
Staphylococcus saprophyticus
Coagulase negative staphylococci
Enterococcus faecalis
Staphylococcus aureus
Streptococcus agalactiae
Citrobacter species
Klebsiella species
Bacillus subtilis speciesIt is used in the treatment of infections caused by these organisms.Many or all strains of the following genera are resistant to nitrofurantoin:
Enterobacter
Proteus
PseudomonasAntibiotic susceptibility testing should always be performed to further elucidate the resistance profile of the particular strain of bacteria causing infection. Special populations
Pregnancy
Nitrofurantoin is pregnancy category B in the United States and pregnancy category A in Australia. It is one of the few drugs commonly used in pregnancy to treat UTIs. It however should not be used in late pregnancy due to the potential risk of hemolytic anemia in the newborn. Newborns of women given this drug late in pregnancy had a higher risk of developing neonatal jaundice.Evidence of 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 100 mg capsules – taken once every 6 hours
Monohydrate/macrocrystals - (Macrobid) – 100 mg 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).
In studies of dogs, the majority of urinary excretion is through glomerular filtration with some tubular secretion. There is also tubular absorption which is increased with urine acidification. However the activity of nitrofurantoin is also pH dependent and mean inhibitory concentration rises sharply with increased pH above 6. Nitrofurantoin cannot be used to treat infections other than simple cystitis. At the concentrations achieved in urine (>100 μg/mL), nitrofurantoin is a bactericide. It is bacteriostatic against most susceptible organisms at concentrations less than 32 μg/mL.Nitrofurantoin and the quinolone antibiotics are mutually antagonistic in vitro. It is not known whether this is of clinical significance, but the combination should be avoided.Resistance to nitrofurantoin may be chromosomal or plasmid-mediated and involves inhibition of nitrofuran reductase. Acquired resistance in E. coli continues to be rare. Nitrofurantoin and its metabolites are excreted mainly by the kidneys. In renal impairment, the concentration achieved in urine may be subtherapeutic. Nitrofurantoin should not be used in patients with a creatinine clearance of 60 mL/min or less. However, a retrospective chart review may suggest nitrofurantoin is not contraindicated in this population. Mechanism of action
Nitrofurantoin is concentrated in the urine, leading to higher and more effective levels in the urinary tract than in other tissues or compartments. With a 100 mg oral dose, plasma levels are typically less than 1 µg/mL while in the urine it reaches 200 µg/mL.The drug works by damaging bacterial DNA, since its reduced form is highly reactive.