Data on nitazoxanide overdosage is not available ^{FDA label}. In studies in rodents and dogs, the oral LD50 was higher than 10,000 mg/kg. One-time oral doses of up to 4000 mg nitazoxanide have been given to healthy adult volunteers without severe adverse effects. Gastric lavage may be appropriate soon after oral administration if overdose occurs. Supportive and symptomatic treatment should also be administered ^{FDA label}.

According to previous studies ^{FDA label}, less than 1% of the patients age 12 years and older participating in clinical trials with NTZ suffered from the following adverse effects:
Systemic: asthenia, fever, pain, allergic reaction, pelvic pain, back pain, chills, fever, flu-like syndrome.
Central Nervous System: dizziness, somnolence, insomnia, tremor, hypesthesia.
Gastrointestinal System: vomiting, dyspepsia, anorexia, flatulence, constipation, dry mouth, thirst.
Urogenital System: discolored urine, dysuria, amenorrhea, metrorrhagia, kidney pain, edema labia.
Metabolic & Nutrition: increased SGPT.
Hemic & Lymphatic Systems: anemia, leukocytosis.
Skin: rash, pruritus.
Special Senses: eye discoloration, ear ache.
Respiratory System: epistaxis, lung disease, pharyngitis.
Cardiovascular System: tachycardia, syncope, hypertension.
Muscular System: myalgia, leg cramps, spontaneous bone fracture.