Health Centres - Viagra
How does it work?
Viagra tablets contain the active ingredient sildenafil, which is a medicine used in the treatment of erectile dysfunction (impotence). Sildenafil is called a phosphodiesterase type 5 inhibitor, and works by preventing the action of a chemical in the body called phosphodiesterase type 5.
An erection is produced via a complex chain of events, involving signals from the nervous system and the release of chemical messengers within the tissues of the penis. One of these chemical messengers is called cyclic GMP.
Cyclic GMP causes the blood vessels in the penis to widen by relaxing a thin layer of muscle found in the blood vessel walls. This allows more blood to enter the penis, which ultimately results in the penis becoming rigid and erect.
Cyclic GMP is normally broken down by another chemical in the body called phosphodiesterase type 5. Sildenafil works by preventing the action of phosphodiesterase type 5, thus stopping the breakdown of cyclic GMP. This means that the blood vessels are kept dilated for longer, improving blood flow to the penis and maintaining an erection.
As sildenafil enhances the actions of the chemical messengers responsible for producing an erection, it will only work once these messengers are present. This means that sexual stimulation is required for it to produce and maintain an erection. The dose should be taken approximately one hour before intended intercourse.
What is it used for?
- Impotence (erectile dysfunction)
Warning!
- This medicine should be taken on an empty stomach, as if it is taken with food it may take longer to work and be less effective.
- This medicine may cause dizziness and visual disturbances. You should make sure you are aware of your reactions to this medicine before driving or operating machinery.
- If you do feel dizzy or lightheaded after taking this medicine you should lie down until the symptoms pass.
- If sildenafil is taken with grapefruit juice, this may cause a modest increase in the blood level of sildenafil and may increase the chance of experiencing side effects.
- If your erection lasts longer than 4 hours (priapism), consult a doctor immediately. Treatment of this condition should not be delayed more than 6 hours, as this can cause damage to the erectile tissue in the penis and irreversible erectile dysfunction.
- This medicine should not be taken more than once a day.
Use with caution in
- Elderly people
Not to be used in
- Women
This medicine should not be used if you are allergic to one or any of its ingredients. Please inform your doctor or pharmacist if you have previously experienced such an allergy.
Side effects
- Headache
The side effects listed above may not include all of the side effects reported by the drug's manufacturer.
How can this medicine affect other medicines?
Sildenafil must not be taken with any form of nitrate medication, as this combination can produce a severe drop in blood pressure that can cause dizziness, fainting, or even a heart attack. Nitrate medications include the following:
- glyceryl trinitrate, isosorbide mononitrate and isosorbide dinitrate (used for angina or heart failure)
Sildenafil should be used with caution in people who are taking alpha-blocker medicines, as the combination may increase the risk of a drop in blood pressure, which may cause dizziness or fainting. This is most likely to occur within four hours of taking the sildenafil. Alpha-blockers include the following:
- alfuzosin
If you are taking an alpha-blocker medicine your doctor will probably start you on a 25mg dose of sildenafil. If you feel dizzy or light-headed after taking the sildenafil you should lie down until the symptoms pass.
Sildenafil should not be taken with other treatments for erectile dysfunction, as the safety of such combinations have not been established.
The following medicines may increase the blood level of sildenafil and you may need a lower sildenafil dose if you are taking any of these:
- cimetidine
NB. The HIV protease inhibitor ritonavir can cause a large increase in the blood level of sildenafil. Sildenafil is not recommended for people taking ritonavir, however if it is used, the maximum dose of sildenafil should under no circumstances exceed 25mg within 48 hours.
- Decreased kidney function
- Decreased liver function
-
Disease involving the heart and blood vessels
- Obstruction of blood flow out of the heart, eg due to narrowing of the aorta (aortic stenosis)
- Bleeding disorders, eg haemophilia
-
Active peptic ulcer
- Conditions such as sickle cell disease, bone cancer or leukaemia in which there is an increased risk of prolonged erections (priapism)
- Physical abnormality of the penis, such as severe curvature, scarring or Peyronie's disease
- Children or adolescents under 18 years of age
- Conditions in which sexual activity is not advisable, for example severe heart disorders such as unstable angina or severe heart failure
- People who have recently had a heart attack
- People who have recently had a stroke
- Low blood pressure (hypotension)
- People taking any form of nitrate medications, eg glyceryl trinitrate
- Severely decreased liver function
- Hereditary degeneration of the back of the eyeball (retina), such as retinitis pigmentosa
- Rare hereditary problems of galactose intolerance, Lapp lactase deficiency or glucose-galactose malabsorption.
- This medicine is not recommended for people who have ever experienced an eye condition called non-arteritic anterior ischemic optic neuropathy (NAION). This condition causes a sudden, partial decrease or loss of vision in one or both eyes due to damage to the optic nerve. It can be temporary or permanent. Talk to your doctor if you have ever experienced this.
Medicines and their possible side effects can affect individual people in different ways. The following are some of the side effects that are known to be associated with this medicine. Because a side effect is stated here, it does not mean that all people using this medicine will experience that or any side effect.
- Flushing
- Dizziness
- Visual disturbances, such as blurred vision, blue tinge to vision
- Nasal congestion
- Awareness of your heart beat (palpitations)
- Indigestion
- Eye pain
- Red eyes
- Abnormal heart beats (arrhythmias)
- Low blood pressure (hypotension)
- High blood pressure (hypertension)
- Fainting
- Nosebleeds
- Rash
- Persistent painful erection of the penis (priapism)
- Chest pain (angina pectoris)
- Bleeding into the tissue of the brain (cerebrovascular haemorrhage)
- Sudden loss of brain function with full recovery within 24 hours (transient ischaemic attack)
It is important to tell your doctor or pharmacist what medicines you are already taking, including those bought without a prescription and herbal medicines, before you start treatment with this medicine. Similarly, check with your doctor or pharmacist before taking any new medicines while taking this one, to ensure that the combination is safe.
- nicorandil
- amyl nitrate (also known as "poppers").
- doxazosin
- indoramin
- prazosin
- tamsulosin
- terazosin.
- erythromycin
- itraconazole
- ketoconazole
- HIV protease inhibitors such as saquinavir.
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