Nitrofurantoin is an excellent [since long used] antimicrobial for treatment and especially prevention of urinary tract infections if natural methods are not helping enough.
I always have been reluctant to use it for an extended time because of [possible serious side effects.

A new Dutch trial showed that 50 mg is just as effective as 100 mg daily.

Guidelines do not distinguish between 50 mg or 100 mg nitrofurantoin as daily prophylaxis for recurrent urinary tract infection (UTI), although 50 mg might have a better safety profile. Our objective was to compare the effectiveness and safety of both regimens.
Daily prophylaxis for recurrent UTI with 100 mg instead of 50 mg nitrofurantoin was associated with an equivalent hazard on UTI or pyelonephritis, and a higher hazard on cough, dyspnoea and nausea. We recommend 50 mg nitrofurantoin as daily prophylaxis.

No differences were observed between 100 mg and 50 mg regarding other adverse or serious adverse events, for example, hepatotoxicity, polyneuropathy or headache.