It is uncertain whether isoniazid poses a carcinogenic risk to humans, but prolonged use of this drug may lead to adverse effects such as liver damage and numbness in the hands and feet. Isoniazid belongs to group 3 carcinogens in the carcinogenicity list issued by the International Agency for Cancer of the World Health Organization, i.e., it is suspected to be carcinogenic to humans, and there is no sufficient evidence to show that there is an increased risk of cancer in patients after taking the drug. However, free hydrazine contained in isoniazid is a carcinogen, and if isoniazid produces free hydrazine in the body for some reason, it may lead to an increased risk of cancer in patients. Therefore, long-term use of this drug is not recommended for patients without indications, and it is contraindicated in patients with allergies, acute liver disease, and psychiatric disorders. Adverse reactions such as nausea, vomiting, fatigue, dizziness, granulocytopenia, rash, tingling of hands and feet may occur after taking isoniazid. Patients should follow the doctor’s instructions to take the drug, and should go to the hospital in time after taking the drug related adverse reactions.