Sildenafil is used to treat erection problems. Find out how to take it safely and possible side effects. Sildenafil is also called Avigra, Silvasta and Viagra.

On this page, you can find the following information:

Type of medicine Also called
  • Belongs to a group of medicines called phosphodiesterase type-5 (PDE5) inhibitors
  • Avigra®
  • Silagra®
  • Silvasta®
  • Vedafil®
  • Viagra®

What is sildenafil?

Sildenafil is used for men with erection problems (erectile dysfunction). It increases blood flow to the penis, to help achieve an erection. To work well, sildenafil needs to be taken 30 minutes or more before sex. This gives it enough time to get into the penis to work. 

Sildenafil belongs to a group of medicines called phosphodiesterase type-5 (PDE5) inhibitors because it blocks the enzyme called phosphodiesterase type-5.  In New Zealand sildenafil is available as tablets.  

How well does sildenafil work?

Sildenafil works well for most men with erectile dysfunction, helping sustain an erection for penetrative sex in about 7 out of every 10 users.

  • Sildenafil does not work so well for men with diabetes, those who have had prostate surgery or have severe erectile dysfunction.
  • If sildenafil doesn't work, other PDE5 inhibitors such as tadalafil or vardenafil may work better.

What can affect how well sildenafil works?

Some things may affect how well sildenafil works. These include:

  • no sexual stimulation
  • trying to have sex too soon after taking sildenafil
  • eating a fatty meal before taking sildenafil
  • drinking alcohol, as it can lessen the sexual response
  • anxiety.

When is it not safe to take sildenafil?

Medical conditions

Sildenafil may not be suitable for everyone. Tell your doctor or pharmacist if you have or have had any of the following:

  • any heart or blood vessel problems
  • leukaemia (cancer of the blood cells)
  • multiple myeloma (a cancer of the bone marrow)
  • any disease or deformity of your penis
  • low or high blood pressure
  • sickle cell anaemia
  • bleeding disorders
  • stomach ulcer
  • liver problems.


Sildenafil should not be used if you are taking medicines called NITRATES.
Nitrates are usually used for chest pain (angina) and other heart conditions.

Nitrates and sildenafil should NOT be taken together, as both medicines together can cause a dangerous drop in blood pressure and collapse. Nitrates come in a variety of forms. Examples of nitrates are:

  • isosorbide mononitrate (Ismo 20®, Ismo 40 Retard®, Duride®)
  • sprays such as Glytrin® or Nitrolingual® spray 
  • patches such as Nitroderm TTS®
  • ointment such Percutol®.

If you are taking a medicine called an ‘alpha blocker’ (such as doxazosin, terazosin, prazosin) your dose may need to be changed.

How to take sildenafil

  • Sildenafil tablets come in different strengths – 25 mg, 50 mg and 100 mg.
  • Dose: The dose can be either 25 mg, or 50 mg or 100 mg. Do not take more than one dose a day. The maximum dose is 100 mg.
  • Timing: Take your dose about 1 hour before you intend to have sex. Sildenafil usually takes 30 minutes to 1 hour to start working. It may take longer to work if you take it with a heavy meal. Sildenafil works best within 4 hours of taking a dose, but it can continue to work for up to 12 hours.

Side effects

Like all medicines, sildenafil can cause side effects, but not everyone gets them. 

Side effects What should I do?
  • Blocked nose
  • Sensation of redness and warmth or burning of the face (called facial flushing)
  • These will pass quite quickly
  • Headache, muscle aches and pains
  • Tell your doctor if troublesome
  • Indigestion 
  • Tell your doctor if troublesome
  • An erection that lasts longer than 4 hours
  • Painful erection
  • Chest pain 
  • Rash or swelling of the face
  • Sudden problems with your eyesight such as loss of vision, in one or both eyes
  • Sudden loss of hearing  
  • Tell your doctor immediately or ring HealthLine 0800 611 116

Learn more

Medsafe Consumer Information (NZ)


Credits: Sandra Ponen, Pharmacist. Reviewed By: Angela Lambie, Pharmacist, Auckland Last reviewed: 05 Jan 2017