According to RxList, the typical dosage of Macrobid for an adult is one 100-milligram capsule every 12 hours for seven days. Macrobid, a brand name of nitrofurantoin, is an antibiotic used to treat urinary tract and bladder infections.
Symptoms of the infection may disappear after just a few days, but the entire prescribed amount of Macrobid should be taken as directed, cautions WebMD. When the full dose of antibiotics is not taken, some bacteria might still remain and continue to grow. Macrobid, like all antibiotics, is most effective when taken at evenly spaced intervals so there is a constant level of the medicine within the body.