The main symptom of erectile dysfunction (ED) is the inability to get and maintain an erection for satisfactory intercourse.

ED should not be confused with ejaculation problems such as premature ejaculation, which is a condition where the process of arousal, orgasm and ejaculation occurs very rapidly.

A significant cause of erectile dysfunction is the narrowing of blood vessels that feed the penis, which restricts blood flow and makes it difficult to achieve an erection.

The active ingredient in Sildenafil is identical to Viagra. Viagra is simply a branded, more expensive version.

Viagra is the brand name that Pfizer (the original company that discovered the chemical and marketed this type of medication) gave to the product. Once Pfizer's patent expired in 2013, other manufacturers aside from Pfizer were able to produce and sell medication containing sildenafil, but still are not allowed to use the Viagra brand name.

Sildenafil works by expanding the blood vessels that feed the penis - this increased amount of blood allows for a stronger erection when you are sexually aroused.

As a general rule you should take sildenafil tablets up to 1 hour before having sex. If the medication works correctly, when you are sexually aroused you will be able to get and maintain an erection.

You should be able to achieve a satisfactory erection for 4-6 hours after taking a Sildenafil tablet. Your erection will go down after ejaculation.

You should only take one tablet in a 24-hour period.

Sildenafil (and other ED treatments) are not always effective for everyone the first time they take them. You should try a tablet on up to 8 different occasions before deciding that it is ineffective. At this point, you can opt to either increase the strength to 100mg or speak to your doctor about alternative treatment options.

The difference is the amount of active ingredient - if you've never had ED treatment before, it's recommended that you begin with 50mg, and only switch to 100mg if this does not have an effect after 8 separate attempts. You can change your strength at any time.

No - clinical studies have shown that doses above 100mg of Sildenafil are no more effective at producing an erection but significantly increase the chances of side effects. You should not take more than 100mg of sildenafil in a 24-hour period.

We've teamed up with one of the UK's leading online pharmacies - regulated by the CQC and GPhC. Their team of fully-qualified doctors and pharmacists write and dispense all of our medication prescriptions.

Yes - if they think it's safe, our doctors will write a private prescription for you after you've answered our confidential online medical questionnaire. Your medication will then be sent straight out to you in the post - a fast, convenient and discreet service without any awkward doctors appointments required.

Yes - as long as your answers to our online medical questionnaire are accurate.

The starting dose for most men is 50mg. If 50mg does not have an effect after 8 separate attempts, then switch to 100mg. If you have existing heart conditions, you should talk to your doctor about starting sildenafil - they may decide that 25mg is a better option for you.

You should avoid the following: Nitrates (glyceryl trinitrate spray & isosorbide mononitrate or dinitrate), Poppers (amyl nitrate), Protease inhibitors (HIV treatment) and Alpha-blockers (doxazosin, prazosin, or terazosin).

If you have questions about using Sildenafil alongside any other medication please feel free to contact our Pharmacists here.

As all ED treatments work in a similar way, you should not take any other ED treatments (such as Viagra, Cialis, Tadalafil, or Levitra) within 24 hours of sildenafil. Always speak to your doctor or pharmacist before seeking alternative ED treatments.

Common side effects happen in more than 1 in 100 people. If you get these side effects, you can keep taking the medicine, but tell your doctor or pharmacist if these side effects bother you or don't go away. Common Side effects may include:

• Headache
• Feeling sick
• Hot flushes, including facial flushing
• A colour tinge to your vision or blurred vision
• Stuffy nose
• Dizziness

Serious side effects are rare and happen in less than 1 in 1,000 people. Stop taking Sildenafil and tell a doctor straight away if you experience a serious side effect. Serious side effects may include:

• Chest pains - if this happens during or after sex, get into a semi-sitting position and try to relax, do not use nitrates to treat your chest pain
• Prolonged and sometimes painful erections - if you have an erection which lasts for more than 4 hours, contact a doctor immediately
• A sudden decrease or loss of vision
• A serious skin reaction - symptoms may include fever, severe peeling and swelling of the skin, blistering of the mouth, genitals and around the eyes
• Seizures

You can find all the information you need on Sildenafil in the Patient Information Leaflet ( https://www.tevauk.com/mediafile/id/39966.pdf). We include one in every pack and suggest you give it a read before you take the medicine.