Slow acetylators of isoniazid are more prone to develop :

Acetylator status of a person determines the response of drugs metabolized by acetylation (e.g. isoniazid, sulfonamides, procainamide and hydralazine). Isoniazid is metabolized to acetyl-isoniazid and then to acetyl hydrazine. Accumulation of isoniazid is responsible for peripheral neuropathy whereas acetylhydrazine accumulation may cause hepatotoxicity. Thus, slow acetylators will not be able to metabolize the drug quickly and there can be an accumulation of parent drug (isoniazid in this case), leading to peripheral neuropathy. On the other hand, fast acetylators are more prone to develop hepatotoxicity.