Sildenafil and tadalafil work through the same erection pathway but differ in timing, duration and side effects. This guide also covers vardenafil and avanafil.
Sildenafil and tadalafil are both PDE5 inhibitors used to treat erectile dysfunction. Sildenafil is usually chosen for on-demand treatment with a shorter period of action. Tadalafil can work for up to 36 hours and is available as either on-demand or low-dose daily treatment for suitable men. Neither is universally better: medical safety, timing, food, side effects and individual response determine the best option.
The doses below describe common licensed starting approaches in adult men, not an individual recommendation. Kidney or liver function, age-related frailty, interacting medicines, response and adverse effects may require a different dose or make a medicine unsuitable.
| Medicine | Usual licensed starting approach | Timing and duration | Food and practical differences |
|---|---|---|---|
| Sildenafil (Viagra) | 50 mg on demand; the prescribed dose may be 25, 50 or 100 mg | Usually taken about one hour before sexual activity; shorter acting than tadalafil | A meal, particularly a high-fat meal, can delay onset; temporary visual colour change is more characteristic than with tadalafil |
| Tadalafil (Cialis) | 10 mg on demand, or 5 mg once daily in selected men; lower or higher licensed strengths are used according to response and tolerability | On-demand treatment is taken at least 30 minutes before sexual activity and may remain effective for up to 36 hours | Can be taken with or without food; back pain and muscle ache are more characteristic |
| Vardenafil | 10 mg on demand; licensed film-coated doses range from 5 to 20 mg | Usually taken 25 to 60 minutes before sexual activity; relatively short acting | A high-fat meal may delay the film-coated tablet; formulation-specific instructions apply to orodispersible tablets |
| Avanafil (Spedra) | 100 mg on demand; the prescribed dose may be 50, 100 or 200 mg | Usually taken 15 to 30 minutes before sexual activity | Food may delay onset; important interactions with strong CYP3A4-inhibiting medicines apply |
Sexual stimulation releases nitric oxide in erectile tissue. This increases cyclic guanosine monophosphate (cGMP), which relaxes smooth muscle and allows blood to enter the penis. PDE5 normally breaks cGMP down. Blocking PDE5 allows the signal to last longer and can make an erection easier to attain and maintain.
The medicine amplifies an arousal-dependent process. It does not produce a continuous erection during its full period of activity, and it will not correct low libido by itself.
Sildenafil has extensive clinical use and is available as a generic medicine. The current UK Summary of Product Characteristics gives a usual adult dose of 50 mg taken as needed approximately one hour before sexual activity, with adjustment to 25 or 100 mg according to efficacy and tolerability. The maximum recommended frequency is once daily.
It may suit a man who:
A meal can delay its onset, and this is more likely after a large or high-fat meal. Excess alcohol can make an erection harder to achieve and may increase dizziness. Common adverse effects include headache, flushing, indigestion, nasal congestion, dizziness, nausea and temporary visual disturbance.
Tadalafil has a longer half-life and an on-demand response window of up to 36 hours. This does not mean that an erection lasts for 36 hours. It means an erection response may be supported during that period when sexual stimulation occurs.
The current UK product information describes 10 mg before anticipated sexual activity as the usual on-demand starting dose, with 20 mg or a lower dose considered according to response and tolerability. For men expecting frequent use, a clinician may consider a once-daily regimen, usually 5 mg and sometimes 2.5 mg, with periodic review of whether daily treatment remains appropriate.
Tadalafil may suit a man who:
Food does not materially restrict on-demand timing. Common adverse effects include headache, indigestion, flushing, nasal congestion, back pain and muscle aches. Because tadalafil remains active for longer, its interactions must be considered over a longer period.
Vardenafil and avanafil use the same PDE5 pathway and can be considered when clinically appropriate. Vardenafil’s usual licensed starting dose is 10 mg around 25 to 60 minutes before sex. A high-fat meal may delay the film-coated formulation. Avanafil’s usual starting dose is 100 mg about 15 to 30 minutes before sex; food may also delay onset.
Formulations are not always interchangeable. For example, a vardenafil orodispersible tablet has specific instructions and is not simply equivalent to the same-strength film-coated tablet. Follow the leaflet for the product actually supplied.
The EAU concludes that PDE5 inhibitors as a group significantly improve erectile function and that there is no demonstrated overall efficacy difference among the four medicines. Individual men can still respond or react differently. A medicine may appear more effective because its timing better fits sexual activity, it was taken correctly, side effects were lower or anxiety was reduced.
The choice can be framed around practical priorities:
Price or brand should not replace a clinical safety check.
Do not take sildenafil, tadalafil, vardenafil or avanafil with any organic nitrate or nitric-oxide donor. Examples include GTN spray or tablets, isosorbide mononitrate, isosorbide dinitrate and amyl nitrite “poppers”. Nicorandil also has nitrate-like properties and must not be combined with a PDE5 inhibitor.
The combination can cause an unpredictable and dangerous fall in blood pressure. It remains contraindicated even if a nitrate is used only occasionally. Tell the prescriber if you carry GTN but have not used it recently.
Riociguat, used for certain forms of pulmonary hypertension, must not be combined with a PDE5 inhibitor because symptomatic hypotension can result.
PDE5 inhibitors are not suitable when sexual activity itself has been judged unsafe. Examples can include unstable angina, uncontrolled blood pressure, severe or unstable heart failure, a recent heart attack or stroke, and some serious rhythm disorders. Exact restrictions differ between products and clinical circumstances.
Stable cardiovascular disease does not automatically exclude ED treatment. It does require an assessment of exercise symptoms, cardiac risk and medicines. Do not stop nitrate treatment in order to obtain an ED tablet without agreement from the clinician managing the heart condition.
Additional cautions or contraindications may apply with:
The full interaction list is product-specific. Provide all prescribed medicines, pharmacy products, supplements and recreational substances during assessment.
Most adverse effects are mild and transient, but serious reactions can occur.
Do not drive or operate machinery if vision is affected or you feel dizzy. Persistent or troublesome symptoms should be discussed with a pharmacist or prescriber.
If more than the prescribed or labelled dose has been taken, contact NHS 111 even if there are no symptoms.
One unsuccessful attempt does not necessarily establish treatment failure. Review:
A clinician may recommend several correctly used attempts, adjust the dose within the licence, or consider a different PDE5 inhibitor. Do not make these changes independently. If oral therapy remains unsuccessful, vacuum devices, alprostadil, injection treatment and penile implants are established alternatives.
Viagra and generic sildenafil contain sildenafil; Cialis and generic tadalafil contain tadalafil. A regulated generic medicine must meet quality standards for its active ingredient and performance. Tablet appearance and inactive ingredients can differ, which may matter if a man has a particular allergy or intolerance.
Counterfeit tablets can contain the wrong dose, no active medicine or undeclared substances. Use a registered pharmacy. For online care, check Great Britain pharmacies on the General Pharmaceutical Council register and Northern Ireland pharmacies on the Pharmaceutical Society of Northern Ireland register. Check that the clinical service and prescriber are appropriately regulated as well.
Many men taking antihypertensives can use a PDE5 inhibitor, but the combination may lower blood pressure further. Alpha-blockers need particular care. Nitrates and nicorandil are different: they are contraindicated, not simply a combination that needs monitoring.
PDE5 inhibitors are treatments for erection quality, not infertility. BAUS states that they do not improve libido, ejaculation or fertility. A man trying to conceive who has erection difficulty should still discuss all medicines and the wider fertility assessment with a clinician.
