Final answer:
A significant side effect of alpha-1 antagonists like Prazosin, Doxazosin, and Terazosin is postural hypotension, which is a drop in blood pressure upon standing up leading to dizziness or fainting.
Step-by-step explanation:
A significant side effect of using alpha-1 antagonists such as Prazosin, Doxazosin, and Terazosin is a condition known as postural hypotension, also referred to as orthostatic hypotension. This side effect entails a sudden drop in blood pressure upon standing up, which can lead to dizziness or fainting. While the question mentions other severe side effects such as insomnia, depression, prostate enlargement, impaired heart function, testicular atrophy, and the risk of colon cancer, osteoporosis, heart disease, macular degeneration, and possibly depression, these are not typically associated with the use of alpha-1 antagonists. In fact, alpha-1 blockers are often used to treat prostate enlargement. It is important to monitor for postural hypotension when starting or increasing the dose of these medications.