What is Viagra? An overview of the 'blue pill'
Viagra is a treatment for erectile dysfunction (ED), first launched by Pfizer in 1998. Its active ingredient is Sildenafil.
Pfizer's patent on Viagra expired in June 2013. This means any government-regulated manufacturer can now produce Sildenafil as a generic medicine. Generic Sildenafil contains the same active ingredient and works in the same way as branded Viagra. The tablets may differ in appearance and price.
At IQ Doctor, you can choose branded Viagra or buy generic Viagra online. Select your preferred treatment from the drop-down menu above. Then complete a free, confidential online consultation for review by one of our UK-registered prescribers.
How does Viagra work?
Viagra helps men, or people assigned male at birth, get and keep an erection long enough for sex. It works by increasing blood flow to the penis.
The active ingredient in Viagra is Sildenafil, which belongs to a group of medicines called phosphodiesterase type 5 (PDE5) inhibitors. Sildenafil blocks an enzyme in the penis that controls blood flow. When you're sexually aroused, this increases blood flow to the penis and helps you get and keep an erection.
Is Viagra or Sildenafil suitable for me?
Sildenafil is suitable for many adult men with erectile dysfunction, but it isn't right for everyone. Your prescriber will review your health and the medicines you take before approving treatment.
Don't take Sildenafil with nitrate medicines, amyl nitrite ('poppers') or riociguat. These combinations can cause a dangerous fall in blood pressure.
Tell your prescriber if you:
Have heart or blood pressure problems
Are allergic to Sildenafil or any of its ingredients
Have recently had a heart attack or stroke
Have an inherited eye condition, or have had sudden vision loss in the past
Have kidney (renal) or liver disease
Have sickle cell disease, leukaemia or multiple myeloma
Have a bleeding disorder or stomach ulcer
Take any other medicines or erectile dysfunction treatments
Always give accurate information in your consultation, so your prescriber can decide whether Sildenafil is suitable for you.
How to use Viagra or Sildenafil
The usual starting dose is 50mg, taken about one hour before sexual activity. A prescriber may recommend 25mg or 100mg instead, depending on your individual circumstances, how well it works and how well you tolerate it.
Viagra starts to work 30 to 60 minutes after you take it, and its effects last 4 hours on average.
If 50mg doesn't work well enough, a prescriber may review how and when you take it. They may also consider adjusting your dose to 25mg or 100mg, depending on how you respond and any side effects. Don't increase your dose without the right medical advice.
Sexual arousal is essential for the treatment to work. Viagra is most likely to be effective if you take it an hour or two before sexual activity. It won't have any effect without sexual arousal, and its effects wear off after around 4 hours.
Avoid large or high-fat meals, as they can delay how quickly Viagra starts to work. Avoid excessive alcohol, which can make it harder to get an erection, and avoid grapefruit and grapefruit juice.
Buy generic Viagra online with IQ Doctor
If you're looking to buy generic Viagra, IQ Doctor makes the process simple, discreet and fully clinician-led. Every order is reviewed individually by one of our UK-registered prescribers, not an automated system.
Once approved, your treatment is dispensed by our pharmacy, which is regulated by the General Pharmaceutical Council (GPhC). It's sent out the same day in plain, discreet packaging. Our UK-based clinical team is here to support you throughout your treatment, free of charge.
Not sure Viagra is right for you? Take a look at our full range of erectile dysfunction treatments.
Ready to get started? Begin your free consultation. Subject to clinical approval, we'll deliver your treatment discreetly to your door.
What are the potential side effects of Viagra and Sildenafil?
Viagra and Sildenafil are generally well tolerated, but side effects can happen. These are usually mild and short-lived.
Common side effects include:
Headaches: these are relatively common and can be treated with over-the-counter painkillers, such as paracetamol
Flushing: this may mean redness of the skin, usually on the cheeks or neck
Indigestion (dyspepsia): this can be treated with indigestion medicines
Changes to your vision
Nasal congestion: this can be relieved with inhalers containing menthol or eucalyptus
Dizziness
Speak to a doctor or pharmacist if a side effect bothers you or doesn't go away. Don't drive or use machinery if you feel dizzy or notice changes to your vision.
Serious side effects are rare. Get urgent medical help if you have chest pain, sudden loss of vision or hearing, a severe allergic reaction, a seizure, or an erection that lasts more than 4 hours, whether or not it's painful.
A prolonged and sometimes painful erection is called priapism. It's rare, but without treatment it could permanently damage the penis.
For a full list of side effects, read the Pfizer Viagra Patient Information Leaflet.
Want to find out more about what Viagra (Sildenafil) does and how to use it? We've answered some common questions below.
Is Viagra safe to use?
Viagra is generally a well-tolerated and effective ED treatment for adult men of any age. It's a prescription-only medicine, so only your doctor can decide if it's the right treatment for you, after a full review of your health.
How quickly does Viagra take effect?
Viagra usually starts to work 30 to 60 minutes after you take it. It can take longer to start working if you take it with a heavy meal. The fattier the meal, the longer it takes.
Take Viagra about an hour before you plan to have sex, and try to take it on an empty stomach.
How long do the effects of Viagra last?
Sildenafil usually starts working 30 to 60 minutes after you take it. When you're sexually stimulated, it can help you get and keep an erection. Its effects may last for several hours, but this doesn't mean you'll have an erection throughout that time.
When does Viagra not cause an erection?
If Viagra doesn't give you an erection, it might be because of:
Not being sexually aroused
Starting sexual activity too soon after taking it (it usually takes 30 to 60 minutes to start working)
Waiting too long after taking it to have sex (its effects typically last around 4 hours)
Not taking a high enough dose of Viagra for an erection
A psychological cause of ED, such as problems with sexual arousal or confidence
What is considered to be a standard dose of Viagra?
Viagra comes in different doses. The lowest dose is 25mg and the highest is 100mg. Your doctor will decide the right dose for you. Men with mild to severe ED commonly benefit from a 50mg dose.
How often can I take Viagra?
Viagra is meant to be taken as needed, about an hour before you plan to have sex. Don't take more than one dose in 24 hours, even if you plan to have sex more than once in a day.
If you'd like a longer-acting ED treatment, talk to your doctor about switching to an erectile dysfunction medicine that lasts longer, such as Cialis (Tadalafil).
Can Viagra be purchased over the counter?
Viagra is a prescription-only medicine, so you need a prescription from a doctor before you can buy it from a pharmacy. Viagra Connect 50mg is available over the counter.
Can I take Viagra or Sildenafil alongside other medications?
Don't take Viagra if you take any medicines known as nitrates. Taking Viagra with nitrates can cause a sudden, severe drop in blood pressure, which is a serious health risk.
If you take nitrates, tell your doctor when you discuss Viagra for ED. Your doctor should also know about any other medicines you take.
Should you avoid any food or drink while taking Viagra?
You can generally eat normally while taking Sildenafil, but avoid grapefruit and grapefruit juice. A large meal, especially one high in fat, may delay when the medicine starts to work. Avoid excessive alcohol, as it can make it harder to get an erection.
Are there other treatment options for ED?
You can use Viagra on its own, or combine it with lifestyle changes to lower your chances of ED coming back. Simple lifestyle changes that can improve ED include:
Keeping a healthy body mass index (BMI)
Stopping smoking
Stopping or cutting down on alcohol
Avoiding recreational drugs
Exercising daily
Reducing stress in your daily life
There are also other treatments for ED to consider, including Cialis (Tadalafil), Spedra (Avanafil) and Levitra (Vardenafil).
Our Clinical Authors
This page has been written by a registered UK pharmacist and reviewed by our clinical team to ensure it is accurate, clear and reflects current clinical guidance.
Last medically reviewed: 06 November 2026
Last updated: 06 November 2026