What is a possible reaction a patient may have to ketoconazole for Cushing's disease?

Ketoconazole is often used in the treatment of Cushing's disease due to its ability to inhibit steroidogenesis. One of the notable side effects of ketoconazole is that it can indeed lead to headaches in some patients. This happens due to various mechanisms, including its effects on hormone regulation and the central nervous system. Headaches can also be a nonspecific reaction to many medications, reflecting an individual's sensitivity to the drug or its effects on neurotransmitter balance.

While the other listed reactions—weight gain, decreased potassium levels, and insomnia—may occur with various medications, they are not typical reactions specifically associated with ketoconazole in the context of treating Cushing's disease. Weight gain may be more relevant in the context of long-term steroid therapy rather than ketoconazole use. Insomnia does not have a direct link as a common side effect of ketoconazole, and while the medication does impact steroid levels, significant changes in potassium levels are not a well-documented reaction associated with it. Thus, a headache is the most appropriate choice regarding the possible reaction to ketoconazole for Cushing's disease.