Nitazoxanide 200 mg is typically prescribed for 3 days when treating infections caused by Giardia lamblia or Cryptosporidium parvum. For these conditions, the usual dosage is 500 mg taken orally twice daily. For other infections, the duration may vary based on the specific diagnosis and severity. It’s important to follow the healthcare provider's instructions regarding dosage and duration to ensure effective treatment and prevent resistance. If symptoms persist or worsen after completing the prescribed course, it's essential to consult a healthcare professional for further evaluation and potential alternative treatments.