Nitazoxanide 500mg typically starts working within a few hours to a couple of days, depending on the infection being treated. For viral gastroenteritis or parasitic infections like Giardia or Cryptosporidium, noticeable symptom relief usually occurs within 24 to 72 hours. The full course of nitazoxanide 500mg should be completed as prescribed by a doctor, even if symptoms improve earlier, to ensure the infection is fully eradicated. The medication is best taken with food to enhance absorption and effectiveness. If symptoms persist beyond a few days, consult a healthcare provider for further evaluation and possible alternative treatment.