Updated over a week ago

Sildenafil dos and don’ts

First of all, thank you for choosing Numan. If your prescription request for Sildenafil 50mg or 100mg has been accepted, or if you considering whether this treatment may be suitable for you, we'd like to help you learn how to use it to its best effect, and give you some important information before you start taking it.

Do understand how it works

Sildenafil works by helping to relax and open up the blood vessels in your penis, allowing for increased blood flow. This means that when you are sexually aroused, you can achieve a harder and longer-lasting erection.

If you’re interested in understanding the science behind erections and how sildenafil works in more detail, check out Numan’s Erection Handbook, which has everything you need to know about erections and more.

Don’t take it if...

  • You have had an allergic reaction to sildenafil or any similar medications in the past

  • You have low blood pressure

  • You are taking medications which contain nitrates (usually for chest pain)

  • You are using amyl nitrites (“poppers”)

  • You have recently had a heart attack or a stroke

  • You have a serious heart or liver condition

  • You have a history of non-arthritic anterior ischaemic optic neuropathy (NAION)

  • You have an inherited condition affecting your retinas, such as retinitis pigmentos

Sildenafil may not be suitable for you if you:

  • have a deformity of your penis, such as Peyronie’s disease

  • have stomach ulcers

  • have a bleeding disorder, such as haemophilia

  • have a heart condition, including left ventricular outflow obstruction

  • have sickle cell anaemia, leukaemia or multiple myeloma

Sildenafil should only be used by men over the age of 18 who have erectile dysfunction. It should not be used with other treatments for erectile dysfunction. If you aren’t sure whether you have any of the conditions listed above, please consult your doctor.

Do take it correctly

Taking sildenafil doesn’t result in an instantaneous erection - a little bit of planning is required. Sildenafil starts working an hour after you take it, and the effects last for four to five hours. This means that there’s a window of about three hours when you can plan to have sex - and you’ll need to be sexually aroused for sildenafil to take effect. Your resultant erection should feel harder and last longer than before. Eating a large meal at the same time as taking sildenafil can delay the effects by about an hour.

The recommended starting dose for sildenafil is 50mg/100mg taken once per day, as needed. If you feel the effect of sildenafil is too strong or too weak, feel free to contact us and we can advise you on adjusting your dosage.

You should not take more than one dose of sildenafil within 24 hours.

It’s worth noting that sildenafil doesn’t alter libido or sexual desire. It also doesn’t have an effect on your testosterone levels.

Do be aware of the possible side effects

If you experience any of the following serious (but uncommon) side effects, you should stop taking sildenafil and seek urgent medical attention:

  • an allergic reaction - symptoms include swelling of the lips, throat and eyelids, sudden difficulty in breathing or wheezing, dizziness, collapse

  • chest pains - you should not use nitrate-containing medication to treat this

  • a prolonged erection lasting more than four hours, which may be painful. This is a medical emergency and you should visit A&E as soon as possible.

  • severe skin reactions - swelling, blistering or peeling skin

  • seizures

Some of the most commonly reported side-effects of sildenafil are:

  • headache

  • dizziness, feeling faint

  • nasal congestion/stuffiness

  • feeling hot or flushed, including facial flushing

  • nausea

  • indigestion

  • visual changes (including colour changes, sensitivity to light and blurred vision)

When your sildenafil does arrive (which should be within the next 48 hours), please make sure to read the patient information leaflet enclosed. It contains other important safety information as well as a complete list of possible side effects.

In the meantime, if you’ve found your way to the bottom of this text, you’re probably going to be someone who’s going to be interested in our Erection Handbook, a comprehensive guide to erections, how they can go wrong and more.

In case of an urgent medical problem, please contact NHS 111 via or by calling 111. NHS 111 is available 24 hours a day, 7 days a week.

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