Viagra(Sildenafil) is a vasodilator (that is, a drug that dilates blood vessels), and consequently it lowers the systolic blood pressure (the "top" number in blood pressure measurements) by an average of 8 mmHg. In the majority of patients with heart disease, including most of those being treated with antihypertensive drugs, this is not a problem. Studies have shown that the incidence of side effects (including side effects related to low blood pressure, such as dizziness and fainting) are no higher in users of sildenafil who are also taking antihypertensive drugs
That is because Viagra was a mixture of compounds put together in a drug company to give help to angina sufferers, but the very noticeable side effect was seen. However, they re-branded it for the purpose we see today.