Sildenafil, tadalafil and vardenafil are the three prescription PDE5 inhibitors most UK patients ask about for erectile dysfunction (ED). They share a mechanism, but they are not interchangeable — how quickly each works, how long each lasts, and how each behaves alongside food or alcohol vary in ways that genuinely change how they fit into everyday life. This article walks through the three, side by side, in the way a UK clinician approaches the conversation.
The three PDE5 inhibitors at a glance
Sildenafil is the active ingredient in Viagra, and is also available as a generic tablet and — at a lower 50 mg strength — as the over-the-counter product Viagra Connect for suitable adults. Tadalafil is the active ingredient in Cialis, and is available as a generic tablet and as a daily low-dose option. Vardenafil is the active ingredient in Levitra. All three are PDE5 inhibitors — a class of medicines that blocks the phosphodiesterase type 5 enzyme, which allows a chemical called cyclic GMP to build up in the smooth muscle of the penis and support an erection when there is sexual stimulation.
None of the three cause an erection on their own. They all require sexual stimulation to work. That is a common misunderstanding worth clearing up before comparing timings and doses. For a more detailed background on the class, our article on how PDE5 inhibitors work for erectile dysfunction covers the mechanism in plain English.
| Feature | Sildenafil (Viagra) | Tadalafil (Cialis) | Vardenafil (Levitra) |
|---|---|---|---|
| Class | PDE5 inhibitor | PDE5 inhibitor | PDE5 inhibitor |
| Onset | ~30-60 minutes | ~30 minutes | ~30 minutes |
| Duration | 4-6 hours | Up to 36 hours | 4-6 hours |
| Food sensitivity | Slowed by a heavy/fatty meal | Minimal — food has little effect | Slowed by a heavy/fatty meal |
| Typical on-demand doses | 25 mg, 50 mg, 100 mg | 10 mg, 20 mg | 5 mg, 10 mg, 20 mg |
| Daily low-dose option | No | Yes — 2.5 mg or 5 mg | No |
| UK licensed use | ED (adults) | ED and BPH symptoms | ED (adults) |
| Common side effects | Headache, flushing, indigestion, blocked nose | Headache, flushing, indigestion, back or muscle ache | Headache, flushing, indigestion, blocked nose |
Same class, different pharmacokinetics
All three medicines do the same job: they inhibit PDE5, which lets cyclic GMP accumulate in the smooth muscle of the corpora cavernosa, which in turn allows the blood vessels to relax and the penis to become and stay erect during sexual stimulation. The reason they behave differently is not their target — it is their pharmacokinetics, meaning how the body absorbs, distributes and eliminates them.
Sildenafil and vardenafil have relatively short plasma half-lives, around 3.5-5 hours, and are therefore active for around 4-6 hours in most people. Tadalafil has a much longer half-life — around 17.5 hours — which is why its effect can last up to 36 hours. The longer window is often described as being "ready when you are ready", and it is the reason tadalafil is sometimes referred to as the weekend tablet. That difference in kinetics is also why tadalafil can sit at a low continuous dose without needing constant re-dosing.
Onset and duration compared
Onset is broadly similar across the three — around 30-60 minutes for sildenafil, and around 30 minutes for tadalafil and vardenafil on an empty stomach. That is an approximate window rather than a promise; some people respond earlier, some later, and a heavy meal can push everything back. In practice, planning around a 30-60 minute lead time before intimacy is a sensible starting point for all three.
Duration is where the practical differences become obvious. Sildenafil and vardenafil give a 4-6 hour window. If intimacy happens once or twice per week and is broadly planned, that is often plenty. Tadalafil's up-to-36-hour window is a different lifestyle proposition — it allows for more spontaneous intimacy over a longer period from a single dose, which some people prefer and others find unnecessary. Our article on how long Viagra lasts goes into the sildenafil duration question in more detail.
Food and alcohol interaction
Food sensitivity varies. Sildenafil and vardenafil are both slowed by a heavy or high-fat meal — the peak concentration is lower and comes later. Taking either on a lighter stomach helps a more predictable onset. Tadalafil is much less food-sensitive, which is another reason it is often chosen when timing around meals is difficult or unpredictable.
Alcohol is not directly contraindicated with any of the three, but heavy drinking around dosing can worsen side effects (particularly headache, flushing and postural dizziness) and can independently reduce erectile function. Moderate alcohol is generally acceptable; heavier drinking often is not. A clinician can advise on realistic limits based on your circumstances.
Side-effect profile compared
Because the mechanism is shared, so is the side-effect profile. Common effects across all three include headache, facial flushing, indigestion, nasal congestion and back or muscle ache. Tadalafil is more strongly associated with back or muscle ache; sildenafil and vardenafil more with blocked nose and visual disturbance (a mild bluish tint, transient) at higher doses. Most side effects are mild and settle without action. Our companion piece on Viagra side effects — what to expect is a useful further read.
Serious effects are uncommon but recognised across the class. Priapism (a prolonged, painful erection lasting more than four hours) requires emergency medical attention. Sudden vision or hearing loss is rare but warrants stopping the medicine and seeking urgent review. All three are absolutely contraindicated with nitrate medicines because the combined vasodilator effect can cause a dangerous drop in blood pressure. They also interact with alpha-blockers and some antifungals and antivirals, and are not appropriate in some cardiovascular conditions. A clinician-led screening consultation checks for these before any supply.
Daily vs on-demand tadalafil
Tadalafil is the only PDE5 inhibitor available in the UK as a licensed low-dose daily tablet — typically 2.5 mg or 5 mg once daily — as an alternative to the on-demand 10 mg or 20 mg dose. The daily dose gives continuous coverage and allows intimacy without pre-planning around a tablet. For some people that suits their life better; for others it is unnecessary and on-demand dosing is more appropriate. It is a clinician-led decision that considers frequency of intimacy, comorbidities (tadalafil is also licensed for benign prostatic hyperplasia symptoms, which can be relevant for some men), and how you tolerated on-demand dosing if you have used it.
How UK clinicians match a patient to a molecule
Choosing between sildenafil, tadalafil and vardenafil is an individualised clinical decision. A UK clinician will weigh how often you want to be able to have intimacy, whether you prefer planning around a tablet or a more open window, your comorbidities and any other medicines you take, prior PDE5 experience and tolerability, and cost. Someone who has intimacy once a week and can plan around it may be well served by sildenafil or vardenafil on-demand. Someone who wants more spontaneity, or who has benign prostatic hyperplasia symptoms as well, may fit tadalafil better — on-demand or daily. Cost also matters: generic sildenafil and generic tadalafil are widely available and often the more accessible options, while vardenafil (Levitra) tends to be less commonly stocked in the UK.
None of the three is universally the right answer, and none is universally the wrong one. If you have already tried one and it did not suit you — either because it did not work well enough, or because side effects were uncomfortable — that history is directly relevant to the choice of the next step. Your clinician will advise based on your individual circumstances. Underlying causes of ED (cardiovascular, hormonal, psychological, lifestyle) also matter, and our article on the common causes of erectile dysfunction is worth reading alongside this comparison.
Beyond ED — broader considerations
ED is often a symptom rather than a stand-alone problem. It can be an early sign of cardiovascular or metabolic issues, and it interacts with mood, sleep and relationship dynamics. A PDE5 inhibitor treats the symptom; a consultation should ideally sit within a wider look at cardiovascular risk, testosterone if clinically indicated, sleep and mental wellbeing. That is why a UK-registered pharmacy consultation for ED is not just a tick-box for medicine supply — it is the opportunity to raise anything else that might be relevant. Your clinician will advise based on your individual circumstances.