Sildenafil and tadalafil are the two most prescribed erectile dysfunction treatments in the UK, and for most men they're equally effective. The real difference is time: sildenafil works for around 4–6 hours, while tadalafil stays active for up to 36 hours. Choosing between them is less about which is "stronger" and more about how you want treatment to fit your life — planned or spontaneous.
What they are and how they work
Both belong to a class of medicines called PDE5 inhibitors. Sildenafil is the active ingredient in Viagra; tadalafil is the active ingredient in Cialis. Both are now available as low-cost generics containing exactly the same active ingredients as the brands.
During arousal, the body releases nitric oxide, which triggers a chemical (cGMP) that relaxes blood vessels in the penis and increases blood flow. An enzyme called PDE5 breaks cGMP down. Sildenafil and tadalafil block that enzyme, so blood flow is maintained and an erection is easier to achieve and keep. One thing they don't do: create arousal. Sexual stimulation is still required — these medicines assist the plumbing, not the desire.
The key differences at a glance
| Sildenafil | Tadalafil | |
|---|---|---|
| Also known as | Viagra (brand) | Cialis (brand) |
| Starts working | Usually 30–60 minutes | Usually within 30 minutes to 2 hours |
| Lasts | Around 4–6 hours | Up to 36 hours |
| Food effect | Heavy or fatty meals delay absorption — best on a fairly empty stomach | Largely unaffected by food |
| Dosing style | On-demand only | On-demand, or low-dose daily |
| Typical side-effect notes | Slightly more visual disturbance (e.g. blue-tinged vision) reported | Slightly more back pain / muscle aches reported |
Sildenafil: the planned-ahead option
Sildenafil suits occasions you can see coming. Taken around an hour before sex, ideally not straight after a large or fatty meal, it provides a reliable 4–6 hour window. It's the most-studied ED medicine in history — in use since 1998 — which means its effects and side effects are exceptionally well understood.
Tadalafil: the flexible option
Tadalafil's long duration — up to 36 hours — is why it's nicknamed "the weekend pill". That doesn't mean a 36-hour erection; it means the medicine remains available in your system across that window, so intimacy doesn't need scheduling around a dose. It's also unaffected by food, which removes the meal-timing question entirely.
Tadalafil has a second mode sildenafil doesn't: a low-dose daily tablet. Taken every day, it builds a steady level in the body so no pre-planning is ever needed. Daily dosing tends to suit men who are sexually active twice a week or more, or who simply want ED treatment to disappear into the background. In preference studies where men tried both medicines, a majority favoured tadalafil, largely because of the flexibility — though a substantial minority preferred sildenafil, and personal response is what actually decides it.
Side effects and safety
Both medicines share the same common side effects: headache, facial flushing, blocked nose, and indigestion — usually mild and short-lived. Sildenafil is slightly more associated with temporary visual changes; tadalafil with back or muscle aches (which makes sense given how long it stays in the system).
Two safety rules apply to both, without exception:
- Never combine either medicine with nitrates (such as GTN spray or isosorbide mononitrate for angina) or recreational "poppers" — the combination can cause a dangerous drop in blood pressure.
- Never take sildenafil and tadalafil together. Doubling up doesn't double the effect; it stacks the risks.
A rare but serious side effect of both is a prolonged, painful erection (priapism) — an erection lasting more than 2 hours needs urgent medical attention. This is also why ED medicines are prescription-only or prescriber-supervised in the UK: a proper assessment checks your heart health, blood pressure and other medicines first. ED itself can be an early signal of cardiovascular issues, so the check-up has value beyond the prescription.
So which should you choose?
- Occasional, plannable intimacy — sildenafil is simple, proven and effective.
- Spontaneity matters — on-demand tadalafil's 36-hour window removes the clock-watching.
- Regularly active (2+ times a week) — daily low-dose tadalafil makes treatment invisible.
- Big meals are part of the occasion — tadalafil, since food doesn't blunt it.
- First time trying treatment — many men start with sildenafil and adjust from there; if one doesn't suit you, the other often does.
Our prescribers review every consultation and will recommend the medicine — and dose — that fits your health history and lifestyle. If your first choice doesn't work well, don't give up: dose adjustments and switching between the two resolve the issue for most men.
Frequently asked questions
Is tadalafil stronger than sildenafil?
No — at appropriate doses they're similarly effective. Tadalafil lasts far longer (up to 36 hours vs 4–6), which is a difference in duration, not strength.
Can I take sildenafil and tadalafil together?
No. Combining them significantly increases the risk of side effects, including dangerous drops in blood pressure, with no proven benefit.
Does alcohol stop sildenafil or tadalafil working?
A drink or two is generally fine with either, but heavier drinking makes erections harder to achieve regardless of medication and increases side effects like dizziness.
Why didn't sildenafil work for me?
Common reasons: taking it with a heavy meal, not allowing enough time, insufficient stimulation, or a dose that's too low. It's also worth trying on several occasions before concluding it doesn't work — and if it still doesn't, tadalafil or a dose review often solves it.
References
- NHS. Sildenafil (Viagra): common questions. nhs.uk/medicines/sildenafil-viagra.
- NHS. Tadalafil. nhs.uk/medicines/tadalafil.
- Yuan J, et al. Comparative effectiveness and safety of oral PDE5 inhibitors for erectile dysfunction: systematic review and network meta-analysis. Eur Urol. 2013;63(5):902–912.
- Govier F, et al. A multicenter, randomized, double-blind, crossover study of patient preference for tadalafil or sildenafil. Clin Ther. 2003;25(11):2709–2723.
- British Association of Urological Surgeons. Erectile dysfunction (impotence). baus.org.uk.