Sildenafil Dosage Guide

Dr. Babak Ashrafi

Medically reviewed by

Dr Babak Ashrafi

Last reviewed: 21 Nov 2022

Sildenafil is a prescription drug used to treat erectile dysfunction (ED). It is available as a generic medication and under the brand name Viagra.

Sildenafil improves the blood flow to the penis to help you get an erection and keep it for longer. For sildenafil to work, you need to be aroused, and it may take a few attempts for it to work for you.

There are different dosages available and the right one for you will depend on how you respond to the medication. You may be prescribed a lower or higher dose if you have side effects or are not getting the benefits you want from the medication.

You can also switch to another ED medication that works similarly to sildenafil.

Pack of Sildenafil 50mg dosage

What is sildenafil?

Sildenafil is a drug that belongs to a group of medicines called PDE-5 inhibitors. PDE-5 inhibitors improve blood flow to the penis, making it easier to get and maintain an erection.

You need a prescription to buy sildenafil. Our doctors at ZAVA can quickly assess your situation and give you a prescription for it.

Start an online consultation now to see if Sildenafil is right for you
Get started

How does sildenafil work?

Sildenafil works like the original medicament Viagra for the treatment of erectile dysfunction by relaxing your blood vessels and increasing the blood flow to your penis. Sildenafil will not instantly result in an erection because you still need to be sexually aroused for it to work.

What doses of sildenafil are available?

Sildenafil is available in 25mg, 50mg and 100mg doses., the same doses as the original Viagra. You’ll likely start off with 50mg which is often prescribed when taking sildenafil for the first time. This is to see how your body reacts to the medication and if you get any negative side effects. From here, your doctor can adjust your dose easily if required. You should always take sildenafil as instructed by your doctor or pharmacist for the best results and never adjust your dose without guidance.


The usual starting dose for sildenafil is 50mg, but your doctor will decide your starting dose based on your health and what they think is most suitable for you.

If you start taking 50mg tablets and find that you’re unable to get an erection that lasts long enough for sex, your doctor might need to increase your dose. If you have persistent side effects, such as feeling dizzy, your doctor may want to lower your dose or try a different medication.

In either situation, your doctor will be able to give you advice on what to do. You should not change your own dose of sildenafil unless your doctor has instructed you to.


100mg is the highest dose of sildenafil and your doctor may recommend it if the 50mg dose has little or no effect. If you still find that a 100mg does not work for you, try the following:

  • make sure you’ve taken at least 8 separate doses to give it enough of a chance to work
  • speak to your doctor about swtiching to another erectile dysfunction medication
  • try a non medication option


25mg is the lowest dose of sildenafil available and might be right for you if the 50mg dose is causing side effects, or if you have certain medical conditions. If 25mg sildenafil is not effective and you lose your erection during sex, you may want to consider a smaller daily dose of a different erectile dysfunction medication, such as Cialis Daily.

What is the best dose of sildenafil for me?

To work out the best dose of sildenafil for you, your doctor will need to know about your:

  • current health
  • blood pressure
  • cholesterol levels
  • level of fitness
  • history of medical conditions, such as heart problems or diabetes
  • previous erectile dysfunction treatments you’ve tried

With this, your doctor will be able to start putting together a treatment plan for you, and they will be able to adjust it to your needs.

Once you find a treatment suitable for you, always take it as directed by your doctor for best results. There are also lifestyle changes you can consider making to help you ge the most out of your rteatment.

How to take sildenafil to get the best results

To get the best results when taking sildenafil you should:

  • take the medicine as directed by your doctor or pharmacist
  • make sure you’re sexually aroused
  • avoid fatty foods or heavy meals before taking it
  • avoid drinking too much alcohol while taking it

There may be other factors affecting your sexual performance including:

  • stress
  • tiredness
  • emotional factors
  • lack of sleep

Speak to your doctor if you notice anything different about your health, start any new medications, or you want an alternative. They will be able to work with you to find the best medicine suited to you.

I have taken too much sildenafil. What should I do?

If you take more than the maximum daily dose of sildenafil (100mg), you should speak to doctor or pharmacist as this can make side effects more severe. Taking more than the maximum dose will not make sildenafil more effective.

The most common side effects include headache, flushes, diarrhoea and indigestion. You can also get a painful permanent erection (priapism) if you take too much sildenafil. If this happens, you should seek medical attention immediately.

What are the possible side effects of taking sildenafil?

With any medication, it is possible to get side effects. Side effects vary from person to person and in severity. The stronger the dose, the higher the likelihood you will get side effects.

Moderate to severe liver and kidney dysfunction can also make side effects more likely because it remains in the body for longer and in larger quantities.

More than 1 in 10 men get headaches when taking sildenafil.

Common side effects affect 1 in 10 to 1 in 100 people and include:

  • visual disturbances and changes in colour perception
  • flushed skin
  • stuffy nose
  • indigestion

Uncommon side effects affect 1 in 100 to 1 in 1000 people and include:

  • sleepiness
  • decreased touch sensitivity
  • problems with the eyes
  • dizziness
  • ringing in the ears (tinnitus)
  • dry mouth
  • nausea and being sick
  • heartburn
  • skin rash
  • muscle aches
  • blood in the urine or semen
  • bleeding from the penis
  • chest pain
  • increased heart rate

There are also very rare side effects such as a prolonged, painful erection (priapism), skin peeling (Stevens-Johnson syndrome), cardiac arrhythmias, sudden cardiac death, blindness from damage to the optic nerve, seizures and strokes.

For a complete list of side effects, see the patient information leaflet. Learn more of the differences between Viagra and Sildenafil.

Can sildenafil react with other medicines and food?

Yes. Sildenafil is metabolised by enzymes via the liver and excreted via the kidneys, which means it can interact with other medicines and food.

Interactions with other medicines

Always tell your doctor about any other medications you are taking so they can be sure it is safe to prescribe you sildenafil.

For example, drugs such as ritonavir (used to treat HIV infection) mean that sildenafil stays in the body for longer.

Sildenafil must not be taken at the same time as nitrates (for example, nitroglycerin spray, or ‘poppers’) as this can lead to life threatening drops in blood pressure. For similar reasons, you must also be careful if you take alpha-blockers such as tamsulosin.

Interactions with food

You can take sildenafil with or without food, but a heavy meal can delay its effect. Drinking too much can stop you from getting an erection if you have taken sildenafil.

You should not drink grapefruit juice if you are taking sildenafil, as grapefruit juice stops the enzyme that breaks sildenafil down in your body from working.

Do other erectile dysfunction medications have a higher dosage?

Other erectile dysfunction medications are available in different doses, but a higher dose of one medication does not mean it’s better or will work faster than another medication. Some medications may work better for some people than others.

Alternative medications for erectile dysfunction include:

  • Spedra - This is the fastest acting ED medication that usually starts to work in 15 minutes and lasts 4 hours (available from £29.00)
  • Levitra - tends to work in just over 30 minutes and lasts up to 12 hours (available from £40.00)
  • Cialis - tends to work in 15 to 30 minutes and lasts up to 36 hours (available from £39.00)
  • Cialis Daily - This can be taken every single day, so you’re always ready for sex when you want to be without worrying about taking a pill beforehand. (available from £70.00)
  • Another option is to use a cream instead of taking a pill. Vitaros cream contains 3mg per gram of alprostadil and works in 5 to 15 minutes.

Non medication options include:

  • Penis vacuum pump - this device uses suction to achieve erections
  • therapies - therapies such as CBT can address any emotional factors
  • pelvic floor exercises - this improves the ability to maintain erections

We go through these treatments in more detail in our erectile dysfunction treatment guide. You can fill out an online questionnaire with ZAVA and find what could work best for you today.

Medically reviewed by:
Dr Babak Ashrafi Clinical Lead for Service Expansion
Accreditations: BSc, MBBS, MRCGP (2008)

Babak studied medicine at King’s College London and graduated in 2003, having also gained a bachelor’s degree in Physiology during his time there. He completed his general practice (GP) training in East London, where he worked for a number of years as a partner at a large inner-city GP practice. He completed the Royal College of GPs membership exam in 2007.

Meet our doctors

Last reviewed: 21 Nov 2022

Get £5 off your next order when you sign up to our newsletter.
Sign up today

Authorised and regulated by