This guide explains what prescription sildenafil is, how it works, the 25, 50 and 100 mg doses, how to take it, the side effects and the all-important nitrates contraindication. It draws on the UK Summary of Product Characteristics, the NHS and NICE, and is general information rather than personal advice.
What sildenafil is
Sildenafil is a PDE5 inhibitor licensed for erectile dysfunction in adult men 1. It is the active substance in Viagra, and it is also available as generic sildenafil, which is the same medicine at the same doses 1.
On prescription it is supplied as 25 mg, 50 mg and 100 mg tablets, allowing the dose to be matched to the individual 1. The over-the-counter Viagra Connect is the 50 mg form sold after a pharmacist check; prescription sildenafil adds the lower and higher strengths and a clinician's assessment 1.
Like all PDE5 inhibitors, it treats the symptom of ED rather than its cause, so a prescriber will also consider why the ED is happening, since it can signal underlying health conditions 23.
How it works
During sexual arousal, the body relaxes the blood vessels in the penis so it fills with blood; sildenafil supports that process by blocking the PDE5 enzyme, helping a firmer erection 1. It works only with sexual stimulation and does not increase desire 1.
So it helps the mechanics of an erection when arousal is present, rather than creating one on its own 1. Without stimulation, it will not produce an erection 1.
This is the same mechanism across the PDE5 class; the medicines differ mainly in how fast they act and how long they last, which is covered in our comparison guide 13.
The doses: 25, 50 and 100 mg
The usual starting dose is 50 mg, taken as needed about an hour before sexual activity 1. Depending on how well it works and how well it is tolerated, a prescriber may increase it to 100 mg (the maximum) or reduce it to 25 mg1.
A lower dose such as 25 mg may be used for people who are more sensitive to side effects, who are older, or who have certain health conditions or take interacting medicines 1. The right dose is an individual decision made with a prescriber 1.
It should not be taken more than once in 24 hours, and taking more than the prescribed dose increases side effects without necessarily improving the effect 1. Our per-dose guides on sildenafil 100 mg and sildenafil 25 mg go into each strength 1.
Considering treatment for erectile dysfunction? You can start an assessment with a Cloud Pharmacy clinician, who will review your medical history and confirm whether treatment is appropriate.
How to take it
Sildenafil is taken as needed, about one hour before sex, with water 1. It is not a daily medicine, and it works across a window of a few hours, so timing it before anticipated activity is the usual approach 1.
It can be taken with or without food, but a heavy, high-fat meal can delay its onset 1. Alcohol can reduce its effectiveness and worsen side effects, so moderation helps 12.
If it does not work the first time, that is common; it can take a few attempts, and stimulation is essential 1. If it consistently does not help, a prescriber can review the dose or consider another option 12.
Side effects
The most common side effects are headache, flushing, indigestion, nasal congestion and dizziness1. These are usually mild and reflect the medicine's effect on blood vessels 1.
Visual effects, such as a temporary blue tinge or light sensitivity, can occur 1. Rarely, more serious effects are possible, and the product information lists symptoms that need prompt attention 1.
Seek emergency care for an erection lasting more than four hours (priapism), or for sudden loss of vision or hearing 1. These are uncommon but important to act on quickly 12.
Nitrates and other key interactions
Sildenafil must never be combined with nitrates (such as GTN for angina), because the combination can cause a dangerous fall in blood pressure; this is an absolute contraindication 1. The same applies to nicorandil and to recreational 'poppers' (amyl nitrite) 1.
Alpha-blockers (for blood pressure or prostate symptoms) need caution, as the combination can lower blood pressure 1. Some other medicines, including certain antifungals, antibiotics and HIV medicines, can raise sildenafil levels, which is why a prescriber needs your full medicine list 1.
This is a central reason ED medicines are assessed before prescribing 12. Our guides on nitrates and alpha-blockers cover these in detail 1.
Getting sildenafil safely
Prescription sildenafil should be obtained through a registered pharmacy after a proper assessment, whether in person or through a regulated online service 12. The assessment checks your suitability, your other medicines and whether the ED needs further investigation 23.
Buying sildenafil from unregulated websites is risky, because the MHRA warns that fake ED medicines are common and may contain the wrong dose or harmful ingredients 4. A legitimate supply always comes with an assessment 24.
If you think sildenafil might suit you, you can start an assessment with a clinician, who will confirm whether it is appropriate and at what dose 12. Our guide on buying sildenafil online safely covers what a safe service looks like 4.
Getting the most from sildenafil
A few practical points help sildenafil work as well as it can 1. It needs sexual stimulation to work, so a relaxed setting matters; performance anxiety can blunt the effect, and that is common, especially the first few times 12.
Timing matters too: taking it about an hour before, and avoiding a heavy, high-fat meal or too much alcohol beforehand, gives it the best chance 1. If the first attempt is disappointing, it is worth trying a few times under good conditions before concluding it does not work 1.
If it consistently underperforms at 50 mg, a prescriber may consider 100 mg, or look at whether timing, alcohol, anxiety or an underlying condition is the real issue 12. Conversely, if side effects are troublesome, a lower 25 mg dose may suit better 1.
Sildenafil is flexible precisely because it comes in three strengths, so the dose can be tuned to balance benefit and side effects with a prescriber 13.
ED as a signal worth investigating
Because the blood-vessel health behind erections overlaps with general cardiovascular health, persistent ED can be an early signal of an underlying condition such as diabetes, high blood pressure or heart disease 23. For some men it is the first thing they notice 3.
Psychological factors, stress, sleep, alcohol and some medicines can also contribute, so ED is often best understood in the round 23. This is why an assessment looks at more than just prescribing a tablet 2.
Treating the symptom with sildenafil and looking at why the ED is happening are not alternatives; ideally they go together 23. The NHS encourages seeing a clinician if ED keeps happening 2.
So sildenafil can give effective symptom relief, while a proper assessment makes sure nothing important is being missed underneath 23.
Key safety points to remember
A few safety points are worth holding onto with sildenafil 1. The first is the absolute rule never to combine it with nitrates, including GTN sprays or tablets for angina, nicorandil, or recreational 'poppers', because the blood-pressure drop can be dangerous 1.
The second is to tell whoever prescribes or supplies it about all your medicines and health conditions, since alpha-blockers and some other medicines interact, and certain heart conditions mean it should be avoided or used only on advice 1. Honesty at the assessment is part of the safeguard 12.
The third is to know the red-flag symptoms: an erection lasting more than four hours, or sudden loss of vision or hearing, both need emergency care 1. These are uncommon, but acting quickly matters 1.
And the fourth is to obtain it only from a registered pharmacy after a proper assessment, because the MHRA warns that fake ED medicines are widespread online 42. Following these points keeps a very effective medicine safe to use 1.
Beyond the rules, a little common sense helps 1. Give it a fair trial under relaxed conditions, do not double up doses if the first does not work, and go back to a prescriber if the response is consistently poor or side effects are troublesome, rather than self-adjusting 12. Sildenafil is well established and, used correctly, has a long track record, so the aim is simply to use it the right way and have any ongoing ED properly reviewed 13.
It is also worth knowing that ED is extremely common and that needing a medicine for it is nothing to feel awkward about 23. Many men use sildenafil successfully, and a proper assessment is a routine, confidential part of getting it safely, not a hurdle 12. Approached that way, it is a straightforward and effective option for most men for whom it is suitable 1. The combination of a clear safety framework, three dose strengths and a long track record is what makes sildenafil the medicine most prescribers reach for first when ED treatment is appropriate 13. If you think it might help, a short assessment with a clinician is the safe way to confirm it is right for you and at which dose 12.
Frequently asked questions
What doses does sildenafil come in?
On prescription, 25 mg, 50 mg and 100 mg 1. The usual starting dose is 50 mg, adjusted up to 100 mg or down to 25 mg by response and tolerability, with a maximum of once in 24 hours 1.Is generic sildenafil the same as Viagra?
Yes. Generic sildenafil is the same active substance at the same doses as Viagra; the difference is branding, not the medicine 1.How long before sex should I take it?
About one hour before, with water, as needed rather than daily 1. A heavy, high-fat meal can delay onset, and alcohol can reduce effectiveness 1.Can I take it with nitrates?
No, never. Combining sildenafil with nitrates (such as GTN), nicorandil or 'poppers' can dangerously lower blood pressure and is an absolute contraindication 1. Tell your prescriber all your medicines, including alpha-blockers 1.What are the serious side effects to watch for?
Seek emergency care for an erection lasting more than four hours, or sudden vision or hearing loss 1. Common milder effects are headache, flushing, indigestion and nasal congestion 1.How do I get prescription sildenafil safely?
Through a registered pharmacy after a proper assessment, in person or via a regulated online service 12. The MHRA warns fake ED medicines are common, so avoid unregulated sellers 4.Your next step
Sildenafil is a PDE5 inhibitor for erectile dysfunction, the active ingredient in Viagra, available on prescription in 25, 50 and 100 mg. It is taken as needed about an hour before sex, works only with sexual stimulation, and the usual starting dose of 50 mg is adjusted by response and tolerability. It must never be combined with nitrates, needs caution with alpha-blockers, and common side effects such as headache and flushing are usually mild, while an erection lasting over four hours or sudden vision or hearing loss need emergency care.
Get sildenafil only through a registered pharmacy after a proper assessment, since the MHRA warns that fake ED medicines are common, and an assessment also checks whether the ED needs further investigation. If you think sildenafil might suit you, you can start an assessment with a Cloud Pharmacy clinician to confirm whether it is appropriate and at what dose.
Disclaimer
This guide is for general information only and does not constitute medical advice, diagnosis or treatment. The information here describes general clinical context based on UK regulatory sources cited above; it is not a recommendation for any specific medicine or treatment, which can only be made by a prescriber following individual assessment.
If you are considering treatment, speak to your GP or pharmacist, or arrange a consultation with a Cloud Pharmacy clinician. Prescription-only medicines are issued only after clinical assessment and where appropriate.
If you experience side effects from any medicine, you can report them through the Yellow Card scheme at yellowcard.mhra.gov.uk.
References
- Viagra (sildenafil) SmPC 4.1/4.2/4.3/4.4/4.5/4.8/5.1 (PDE5 inhibitor for ED; 25/50/100 mg; start 50 mg as needed ~1 hour before sex, max once/24h; works only with stimulation; food/alcohol; common side effects; nitrates absolute contraindication incl. nicorandil/amyl nitrite; alpha-blocker and CYP3A4 interactions; priapism >4h, vision/hearing loss)
- Sildenafil (how used; assessment; persistent ED warrants review; ED can signal underlying conditions)
- NICE CKS erectile dysfunction (PDE5 inhibitor class; assessment; underlying causes)
- MHRA #FakeMeds (fake ED medicines common online; buy only from registered pharmacies; no specific product claim)
