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How does Viagra work?

Viagra works by increasing blood flow to the penis when sexually stimulated, 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) the enzyme PDE5 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 tadalafil, vardenafil, and avanafil, support sexual relations when erections are failing.

How to take Viagra

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 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. Taking a higher dose has not been shown to improve results and increases your risk of severe side effects.

Sexual 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).

Viagra dosage

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

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Does Viagra 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
• Sexual 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%

Graph: sildenafil 100mg bioavailability over time, with and without food.

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Viagra warnings

During the assessment process with Dr Fox men are required to complete a medical questionnaire to determine suitability.

Men with the following conditions should not take Viagra without further assessment from their GP:

• Taking medicines called nitrates (often given for chest pain)
• Taking or intend to take amyl nitrite ('poppers')
• Taking riociguat (to treat high blood pressure in the lungs)
• Low or high blood pressure
• Allergies or reactions to past ED treatments
• Recent heart attack (within 6 weeks) or suffer from angina (heart pain on exertion) or any form of heart disease
• History of strokes or mini-strokes (transient ischaemic attack)
• History of vision problems caused by circulation problems
• History in the family of rare degenerative eye disease
• Serious kidney (renal) or liver disease
• Peyronie's disease – angulation or deformity of the penis
• Leukaemia, multiple myeloma or sickle cell disease
• Bleeding disorders (such as haemophilia)
• Current active stomach ulcers (peptic ulcer or gastric ulcer)
• Lactose intolerant (tablets contain lactose)

Dr Fox cannot supply ED treatment to men who have any of the above conditions.

There is a very rare theoretical risk of abnormal heart rhythm if Viagra (sildenafil) is taken at the same time as some other medications - please check this list.

Keep medicines out of the sight and reach of children.

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Possible side effects of Viagra

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 effects 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 (telephone NHS 111 in the UK). This applies particularly to chest pains (possible 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.

Alternative treatment to Viagra

Other oral ED drugs work in a similar way, with similar side effects. Viagra was the first drug in this class to become widely prescribed.

The following oral tablet treatments for ED are available to buy online in various strengths and pack sizes from Dr Fox after completing the same medical consultation:

• Sildenafil (25mg, 50mg, 100mg)
• Viagra Connect (50mg sildenafil)
• Cialis (2.5mg, 5mg, 10mg, 20mg tadalafil)
• Cialis Together (10mg tadalafil)
• Tadalafil (2.5mg, 5mg, 10mg, 20mg)
• Vardenafil (5mg, 10mg, 20mg generic Levitra)
• Spedra (50mg, 100mg, 200mg avanafil)

Non tablet alternatives

Other treatments for ED include topical applications, injections into the penis, surgical devices, penis pumps, and counselling.

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