Never take Viagra if:

If you are taking any prescription or non-prescription medicine, herbal preparation, or dietary supplement

If you have allergies to medicines, foods, or other substances

You are taking any form nitrates medication, e.g. isosorbide, nitroglycerin or nitroprusside.

You are allergic to any ingredient in Viagra.

You should check with your doctor before using Viagra if you have a deformed penis; due to conditions such as Peyronie disease, cavernosal fibrosis.

You have been advised by your doctor to avoid sexual activity because of heart problems. You are using certain recreational drugs known as poppers – amyl nitrate or butyl nitrate. If you have a history of certain eye problems; e.g. retinitis pigmentosa, sudden vision loss, optic neuropathy or macular degeneration.

If you have a history of liver or kidney problems, high or low blood pressure, ulcers, bleeding problems, heart problems or blood vessel problems. If you have a history of heart attack, stroke or life-threatening irregular heartbeat. This is especially important if these have occurred within the past 6 months.