The antituberculous drug isoniazid has weak monoamine oxidase inhibiting properties. Drug and dietary restrictions generally applied to the use of monoamine oxidase inhibitors (MAOI) have not been routinely recommended with its use. Here we report three cases in which antidepressant drugs and isoniazid were co-administered. In two, adverse events, possibly due to an interaction between the drugs prescribed are described. We suggest that the combination of isoniazid and antidepressants be used with caution until further data are accumulated.