Isoniazid causes a multitude of adverse drug events.  Most are, however, very rare and of little practical importance.  The most important ones are hepatitis and peripheral neuropathy.

If not recognized, isoniazid-induced hepatitis may be rapidly fatal.  In case of hepatitis, isoniazid should be stopped immediately.  Re-introduction of isoniazid after the most acute phase is often successful.

Peripheral neuropathy is not uncommon in patients with pre-existing vitamin B6 deficiency or increased requirement (e.g., during pregnancy).  In such patients a supplemental dose of 10 mg per day usually suffices to prevent isoniazid-associated neuropathy.

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