When taking isosorbide mononitrate, it is important to avoid alcohol, medications for erectile dysfunction (such as Viagra), and medications for pulmonary hypertension (like Adempas). Grapefruit juice should also be avoided as it can interact with isosorbide mononitrate. It's best to consult with your healthcare provider about any medications or substances to avoid while taking isosorbide mononitrate to ensure your safety and the effectiveness of the treatment.