Viagra ( Sildenafil )

Weekly dose: the hard facts on Viagra

Senior Lecturer in Pharmaceutics, University of Sydney

Disclosure statement

Dr Wheate in the past has received funding from the ACT Cancer Council, Tenovus Scotland, Medical Research Scotland, Scottish Crucible and the Scottish Universities Life Sciences Alliance for research into platinum drugs. Dr Wheate is a named inventor on two international patents dealing with the drug delivery of anticancer drugs.

University of Sydney provides funding as a member of The Conversation AU.

The Conversation UK receives funding from these organisations

Viagra is one brand name of a drug that is used to treat erectile dysfunction (impotence): the inability to get, and maintain, an erection. It is also used in the treatment of high blood pressure specific to the heart and the arteries that feed the lungs. The generic name of the drug is sildenafil (pronounced sill-den-a-fill).

Like many great scientific discoveries, the benefits of Viagra in treating erectile dysfunction were discovered by accident. The pharmaceutical company Pfizer initially developed the drug in 1989 as a treatment for high blood pressure and angina.

Unfortunately, early clinical trials were not positive. To have the right effect the drug had to be taken three times a day and it gave some of the patients muscle aches. It was also found to interact with nitrates (the standard treatment for angina), which resulted in an unsafe drop in patients’ blood pressure.

Luckily, some patients had reported the delayed side effect of erections. As a result, the focus on Viagra changed to its sexual applications.

After clinical testing, it was approved for use in Australia in 1998, having been approved in the US nine months earlier.

How it works

Viagra works by binding to an enzyme called phosphodiesterase. Viagra binding to the enzyme prevents it from converting the chemical cyclic guanosine monophosphate (abbreviated cGMP) into guanosine triphosphate (GTP).

The effect of a build-up in cGMP is the relaxing of artery walls. This makes the arteries bigger and thus allows more blood to flow into the penis.

However, the drug will not give patients an erection without sexual stimulation. When a man becomes aroused the brain sends a signal to the cells in the penis to release nitric oxide, which turns on the production of cGMP. Without the stimulation to produce nitric oxide, there is no cGMP build-up and no erection.


All forms of the drug are sold as solid oral tablets in doses between 25 and 100 milligrams. For the treatment of high blood pressure, the tablets also come in a 20 milligram dose.

The dose prescribed by the doctor takes into account a patient’s age, underlying cause of their impotence, and their kidney function.

When used to treat erectile dysfunction, patients are directed to take the drug once per day, 30 minutes to one hour before their planned sexual activity. The effects of the drug will take longer to kick in if it is taken with food.

Side effects

One in ten patients will experience the common side effects of Viagra. Side effects include the development of a rash, diarrhoea and an increased susceptibility to urinary tract infections.

A less common side effect is abnormal vision. This includes blurred vision, a sensitivity to light, and/or a blue-green tinge to the patient’s vision called cyanopsia. Higher doses increase the chance of a patient experiencing abnormal vision.

Unusual alternative uses

Other than its approved medical uses, Viagra has also been shown to have alternative applications. For example, researchers have shown adding Viagra to the water of cut flowers makes them last longer.

When they gave just one milligram of Viagra to the flowers they were found to stand up straight for a week longer than flowers that were given ordinary water – a doubling in the flower’s shelf life.

Counterfeit Viagra

Viagra is one of the most counterfeited medicines in the world. Because of its wide appeal many patients attempt to buy it without a prescription. Viagra can be bought from online auction sites, internet pharmacies and even bricks-and-mortar retail stores.

There are numerous risks in taking counterfeit Viagra. Fake tablets can contain no Viagra or too much Viagra. There have also been reports that counterfeit tablets have contained blue printer ink – to give them that genuine Viagra look – illegal drugs like amphetamines, and other unrelated drugs like the antibiotic metronidazole.

The best way to ensure you are not buying counterfeit Viagra is to have a trusted pharmacist fill your script in-store. For those patients who need to have their Viagra script filled over the internet, there are some ways to check whether the medicine you have been supplied is fake.

Look for the writing stamped into the tablets. Does it look crisp and sharp or does it have an out-of-focus look? Also, check their colour consistency; you are looking to see if there are regions that are a darker or lighter blue than the rest of the tablet.

Next, look to see if the tablets feel smooth. Rough or pitted tablets are a sign of fake medicines.

Viagra is supplied in packs of four. If your strip has more than four tablets, it’s counterfeit.

And, finally, Viagra is supplied only as solid tablets. Your tablets are counterfeit if you have been supplied soft tablets, gel tablets or chewable tablets.

If you suspect you have been provided counterfeit tablets, take them to your local pharmacist for checking and disposal.

For the treatment of erectile dysfunction, Viagra comes in three doses (25, 50 and 100 milligrams) with four tablets per packet. The cost to the patient is the same regardless of the dose at $6.20 per packet.

The Australian Pharmaceutical Benefits Scheme subsidises the drug; the cost to the government is between $54 to $82 per packet depending on the brand. Patients can expect to pay more if they buy Viagra over the internet.