Cialis vs Viagra: Which ED medication is right for you?

DoktorABC editorial team

Accurate, up-to-date health information written by our editorial team and reviewed by UK-registered prescribers.

Cialis vs Viagra: Key comparison table

Prices based on UK private prescription data, April 2026. Always confirm current pricing with your provider.

How do Cialis and Viagra work?

Both Cialis and Viagra work by inhibiting phosphodiesterase type 5 (PDE5), an enzyme that breaks down cyclic GMP (a chemical messenger that relaxes smooth muscle and increases blood flow) in the penile arteries. By blocking PDE5, both drugs allow cyclic GMP to accumulate, enhancing the effect of nitric oxide and producing vasodilation in the presence of sexual stimulation. Neither medicine produces an erection independently; both require sexual arousal to be effective.

The key pharmacological difference is their half-life (the time it takes for the body to reduce the drug concentration by half). Sildenafil has a half-life of approximately three to five hours, which is why its effects are relatively short-lived. Tadalafil has a half-life of approximately 17.5 hours, which is why it remains active for up to 36 hours and can be taken as a once-daily low dose to maintain a continuous background effect.

When does each medication start working?

Both Viagra and Cialis have a similar onset of action: most men notice effects within 30 to 60 minutes of taking a dose. However, there are practical differences worth noting.

Viagra's absorption is significantly affected by food. A high-fat meal taken before or alongside Viagra can delay absorption by up to two hours and reduce peak plasma concentrations. For best results, Viagra should be taken on an empty stomach or after a light meal.

Cialis is not meaningfully affected by food, making timing more flexible. It can be taken with or without a meal without significant impact on onset or efficacy.

How long does each medication last?

This is where the two medicines differ most significantly in practical terms.

Viagra is typically effective for four to six hours. This provides a defined window for sexual activity, which suits men who prefer a predictable and time-limited treatment.

Cialis on-demand (10 mg or 20 mg) remains effective for up to 36 hours after a single dose, earning it the informal name "the weekend pill." This extended window removes the need to time the dose closely to sexual activity and allows for greater spontaneity over a longer period.

Cialis daily (2.5 mg or 5 mg) is taken once every day, maintaining a continuous low-level effect. For men who have sex frequently or who find on-demand dosing inconvenient, this option removes timing considerations entirely.

Is Cialis better than Viagra?

Neither medicine is clinically superior to the other in absolute terms. Both produce comparable rates of successful erections in clinical trials, and the choice between them depends on individual lifestyle, preferences, and health profile rather than one being more effective than the other.

Cialis may suit men who:

• Prefer not to plan sexual activity around a medication window
• Have sex more than twice a week (in which case daily Cialis may be more practical than repeated on-demand dosing)
• Find Viagra's food interaction inconvenient
• Have tried Viagra without adequate results (though switching should be guided by a prescriber)

Viagra may suit men who:

• Prefer a shorter, more defined treatment window
• Have sex less frequently and find on-demand dosing straightforward
• Are managing costs, as generic sildenafil is widely available and competitively priced in the UK
• Have tried Cialis without adequate results

Is 20 mg Cialis equal to 100 mg Viagra?

No. These are not equivalent doses, and a direct milligram comparison between the two medicines is not clinically meaningful.

Tadalafil and sildenafil have different molecular structures, different potencies, and different dose-response curves. The maximum licensed dose of tadalafil for on-demand use is 20 mg; the maximum licensed dose of sildenafil is 100 mg. These dose ranges reflect the different concentrations required for each molecule to produce a therapeutic effect, not a difference in strength.

Clinical trials have not established a direct equivalence between any specific Cialis dose and any specific Viagra dose. Both medicines produce comparable clinical outcomes at their respective recommended doses, but comparing 20 mg tadalafil to 100 mg sildenafil as if the numbers were comparable is pharmacologically incorrect.

If you are switching from one medicine to the other, your prescriber will determine the appropriate starting dose based on your treatment history and response, not on a numerical conversion.

Does Cialis make you harder than Viagra?

No. There is no clinical evidence that tadalafil produces firmer or more sustained erections than sildenafil, or vice versa. Both medicines work through the same mechanism and produce comparable rates of successful erectile response in clinical trials.

The subjective experience of one medicine feeling "stronger" than the other is reported anecdotally by some men and may reflect individual variation in pharmacokinetics (how each person's body processes a given drug), psychological factors, or simply the difference in duration. These individual responses are real and worth discussing with your prescriber, but they are not supported by evidence suggesting one medicine is inherently superior in producing erections.

What is the difference between Viagra and Cialis?

To summarise the key distinctions clearly:

• Active ingredient: Viagra contains sildenafil; Cialis contains tadalafil
• Duration: Viagra lasts four to six hours; Cialis lasts up to 36 hours
• Daily use: Cialis has a licensed daily dose option; Viagra does not
• Food interaction: Viagra is affected by high-fat meals; Cialis is not
• Dosing: Different dose ranges reflecting different molecular potencies
• Onset: Both take effect within 30 to 60 minutes

Both are MHRA-licensed PDE5 inhibitors with extensive clinical evidence bases, both require a prescription, and neither is suitable for men taking nitrate medications.

Cialis vs Viagra price in the UK

Both medicines are available in branded and generic forms in the UK. Generic versions contain the same active ingredients at the same doses and are subject to identical regulatory standards.

Prices based on UK private prescription data, April 2026. Prices vary between providers. Always confirm current pricing with your chosen GPhC-registered provider.

For most men, generic sildenafil represents the most cost-effective option for on-demand ED treatment. Generic tadalafil daily is a competitively priced alternative for men who prefer continuous cover.

Side effect comparison: How do they differ?

The side effect profiles of Viagra and Cialis are broadly similar, as both are PDE5 inhibitors. The most common effects with both medicines are headache, facial flushing, nasal congestion, and indigestion.

The most clinically notable difference is that back pain and muscle aches are more commonly reported with tadalafil, particularly at the 20 mg dose, while visual disturbances (a bluish tinge or increased light sensitivity) are more associated with sildenafil.

Both medicines share the same absolute contraindication: they must never be taken with nitrate medications, as the combination can cause a severe and potentially life-threatening drop in blood pressure.

Which is better for spontaneity versus planned sex?

For planned sexual activity, both medicines are appropriate. Viagra's defined four to six-hour window suits men who can anticipate when they will need the medication.

For spontaneity, Cialis has a practical advantage. The 36-hour duration of on-demand Cialis means a single dose taken on a Friday evening remains active through the weekend. Daily Cialis removes timing considerations entirely, providing continuous cover without the need to plan around a dose.

For men who prioritise spontaneity or who find the psychological pressure of timing medication adds to performance anxiety, Cialis daily is frequently the preferred option. For men who prefer a clear, time-limited treatment with no continuous medication in their system, Viagra or on-demand Cialis are more suitable.

When should you see a doctor to decide?

Both medicines require a prescription from a UK-registered prescriber. A clinical assessment is required before either can be issued, covering your full health history, current medications, and any cardiovascular risk factors.

You should speak to your GP or a UK-registered prescriber if:

• You have not previously been assessed for ED and are considering treatment for the first time
• You have tried one medicine without satisfactory results and want to discuss switching
• You have cardiovascular disease, high blood pressure, or take any medication that affects blood pressure
• You take nitrate medications for chest pain (both medicines are contraindicated in this case)
• You are unsure which option is most appropriate for your lifestyle and health profile

DoktorABC connects UK patients with licensed prescribers who can assess suitability for treatment, advise on the most appropriate medicine and dose, and provide ongoing clinical guidance.

Final thoughts

Cialis (tadalafil) and Viagra (sildenafil) are both effective, well-evidenced treatments for ED with broadly comparable efficacy. The decision between them comes down to lifestyle fit rather than clinical superiority. Duration, food interaction, the availability of a daily option, and individual tolerability are the practical factors that most often guide the choice. A conversation with your prescriber, rather than a comparison table alone, is the most reliable way to reach the right decision for your circumstances.

FAQ

The DoktorABC medical advisory board

Dr. Roland Ruiken

Medical advisor, Norway

Dr. Viktor Simunovic

Medical advisor, Croatia

To the medical advisory board