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How does Viagra work?

Viagra works by increasing blood flow to the penis which helps men get and sustain an erection long enough for sex.

The active ingredient in Viagra is sildenafil, which is known medically as a PDE5 inhibitor. Sildenafil inhibits (blocks) an enzyme which regulates blood flow in the penis. PDE5 inhibitors multiply the natural process of arousal and increase blood flow to the penis in situations of sexual stimulation. Sildenafil helps to achieve an erection but relies on natural arousal as a trigger. It, and other PDE5 inhibitors such as tadalafilvardenafil, and avanafil, support sexual relations when erections are failing.

How to take

The usual first starting dose of Viagra is one 50mg tablet, for men who have not used it before, although men over the age of 65 years would normally start on the lowest dose tablet (25mg).

Viagra works in 30 to 60 minutes after it is taken and usually remains active for 4 hours.

If 50mg strength of tablet is highly effective subsequent doses may be reduced down to one 25mg tablet. If a 50mg tablet is ineffective, or the duration of activity is less than 4 hours, the dose can be increased to one 100mg tablet. Do not take Viagra more than once per day, or more than 100mg dose.

Arousal is required for the treatment to work. Viagra is most likely to be suitable if taken in situations where sexual arousal is expected within an hour or two of taking the tablet. Without sexual arousal there will be no benefit and the effect of the tablet will simply wear off after about 4 hours.

Avoid large meals, meals high in fat, excessive alcohol, and grapefruit as these can affect efficacy (effectiveness).


photo of Viagra 25mg pack

Viagra 25mg

Reduced dose recommended when 50mg starting dose is highly effective.

photo of Viagra 50mg pack

Viagra 50mg

Starting dose usually recommended for most men taking Viagra for the first time.

photo of Viagra 100mg pack

Viagra 100mg

Strongest dose pill for when 50mg is ineffective.

Do not take more than 1 dose or more than 100mg in 24 hours, or combine with other erectile dysfunction drugs.

Does it work every time?

A trial of erectile dysfunction medication should be repeated at least 4 times before deciding whether Viagra is working and before requesting a change of dosage or change to another ED medication. As with any treatment, there are times when ED tablets might not work and may not improve or overcome the underlying cause of erectile dysfunction.

Satisfactory erections require the following:

  • Adequate blood supply
  • Arousal
  • Healthy nervous system

If erectile dysfunction is the result of spinal injury or operation, nerves might be damaged. In this case Viagra may help a little but other measures, such as injections or stiffening aids, may be required.

Other causes of erectile dysfunction may require specialist treatment apart from tablets. Some men, whose erectile dysfunction has psychological causes, may benefit from counselling.

The clinical study data reports the following results:

Viagra (sildenafil) dosage Men reporting improved erections
25mg 62%
50mg 74%
100mg 82%

Are there any side effects?

Viagra is remarkably well tolerated and free from significant side effects in the vast majority of men who take it. Viagra increases blood flow which can have adverse affects on other parts of the body, although these are usually mild. Most men who experience side effects carry on treatment regardless.

Slightly over 1% of men taking Viagra notice a bluish or yellowish discolouration of their vision. This is not in itself a cause for concern, and usually does not interfere with normal activity. It wears off after a few hours.

The most commonly reported adverse reactions in clinical studies were:

  • Headache – very common and can be treated with simple painkillers such as paracetamol.
  • Flushing (redness of the skin, typically over the cheeks or neck).
  • Indigestion (dyspepsia) – can be treated with indigestion remedies.
  • Visual disorders/distortion.
  • Nasal congestion – can be treated with menthol or eucalyptus inhalers.
  • Dizziness – if affected it is recommended not to drive or operate machinery.

In the rare event of serious side effects seek immediate medical advice. This applies particularly to chest pains (possibly heart attack), loss of vision, and a prolonged and painful erection.

Priapism is the medical name for prolonged painful erections which have the potential to cause permanent damage to the penis. Priapism is rare. A persistent and painful erection of more than 2 hours requires immediate emergency medical attention.

The full range of side effects is also listed in the patient information leaflet supplied with the tablets


25mg, 50mg, 100mg


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