Off-label usage accounts for something like 40% of all medication.
Erectile dysfunction was an off-label use of viagra (sildenafil). It was a cardio-vascular drug first.
It is FDA approved to treat pulmonary hypertension (high pressures in the vessels in the lungs). So probably not off-latel.
Viagra for ED is actually off label.
“Sildenafil, the chemical name for Viagra, is an artificial compound that was originally synthesized and studied to treat hypertension (high blood pressure) and angina pectoris (a form of cardiovascular disease).”