You should not use  isoniazid if you have active liver disease, or if you have taken isoniazid in the past and it caused liver problems, fever, chills, joint pain, or severe allergic reaction.
Serious and sometimes fatal liver problems may occur during treatment with isoniazid or after you stop taking this medicine, especially if you are between the ages of 35 and 65. Your liver function may need to be checked every month while you are taking this medicine.
Call your doctor right away if you have: nausea, upper stomach pain, loss of appetite, and feeling weak or tired.
Avoid drinking alcohol. It may increase your risk of liver damage while you are taking isoniazid.
Before taking this medicine
You should not use isoniazid if you are allergic to it, or if you have:
active liver disease;
a history of severe allergic reaction to isoniazid;
a history of hepatitis or other liver problems caused by taking isoniazid; or
a history of severe isoniazid side effects such as fever, chills, or joint pain and swelling.
To make sure isoniazid is safe for you, tell your doctor if you have:
a history of liver disease;
kidney disease;
nerve problems that cause pain, weakness, or numbness;
diabetes;
HIV or AIDS;
if you drink alcohol daily;
if you are malnourished;
if you use any drugs that are injected; or
if you have ever had to stop taking isoniazid for any reason.
If you are 35 years or older, your doctor will check your liver enzymes before you start treatment, to make sure you can safely use isoniazid.
Serious and sometimes fatal liver problems may occur during treatment with isoniazid or after you stop taking this medication, even months after stopping. The risk of liver problems is highest in adults between the ages of 35 and 65.
Serious liver problems may be more likely to occur in women, especially after childbirth, or in women of Hispanic or African-American ancestry. Ask your doctor about your specific risk.
It is not known whether this medicine will harm an unborn baby. Tell your doctor if you are pregnant or plan to become pregnant.
Talk to your doctor if you will be breast-feeding a baby during your treatment with isoniazid. This medicine can pass into breast milk, but it will not treat or prevent tuberculosis in the nursing infant.
