Cialis vs Viagra: Differences & Risks

Cialis or Viagra: what should you discuss with a doctor?

Cialis (tadalafil) and Viagra (sildenafil) are authorised treatments for erectile dysfunction (ED). Both are phosphodiesterase type 5 (PDE5) inhibitors. Choice depends on your health, other medicines and preferences; neither is suitable for everyone. This comparison is information, not a prescription or a promise of benefit.

Related information: Cialis, Viagra and erectile dysfunction. A page listing does not establish country-specific access.

How they work

Sexual stimulation triggers nitric oxide signalling and production of cGMP, which helps relax blood vessels in erectile tissue. PDE5 normally breaks down cGMP. Inhibiting it supports the response to sexual stimulation; this does not mean every case of ED is caused by unusually fast breakdown. ED can have several contributing causes.

Neither medicine produces an erection without sexual stimulation. Their related mechanism does not make their risks, doses or interactions identical. See the Viagra product information.

Duration and onset

Medicine	Product-information context	Practical limit
Cialis	On-demand use from at least 30 minutes before activity; response may be possible up to 36 hours after a dose	This is a response window, not a continuous erection or guaranteed effect
Viagra	Usually taken about one hour before activity; its action is shorter than tadalafil's	Onset, response and duration vary between people

Tadalafil and sildenafil have different elimination profiles, but half-life alone cannot predict an individual's benefit or the duration of side effects. Do not take another tablet simply because a response has not occurred.

Food and alcohol

Food can delay sildenafil absorption and its onset. Tadalafil can be taken with or without food. This does not mean alcohol is risk-free: drinking can affect erections and blood pressure, and tadalafil's information includes alcohol-related hypotension warnings. Discuss alcohol use and follow the actual leaflet rather than assuming one medicine makes a meal or drinking pattern safe.

Daily dosing and BPH: keep the indication clear

Cialis regimen for ED	Labelled dose examples	Who decides?
On demand	10 mg or 20 mg	The prescriber selects the appropriate regimen
Once daily	5 mg, potentially reduced to 2.5 mg for tolerability	May be considered for frequent anticipated use, such as at least twice weekly; suitability needs regular review

These examples describe the label, not your dose. Daily tadalafil does not guarantee readiness or response. The Cialis 5 mg indication also includes signs and symptoms of benign prostatic hyperplasia (BPH), including ED with BPH; the lower daily ED dose must not be treated as an equivalent BPH regimen. See Cialis sections 4.1–4.2.

Viagra's ED regimen is on demand. This is not a substance-wide claim that sildenafil has no other indication or scheduled regimen: other sildenafil products, including Revatio, are used for pulmonary arterial hypertension (PAH). PAH doses and indications must not be transferred to ED. See EMA's Revatio overview.

Possible side effects

Headache, flushing, indigestion, nasal symptoms, dizziness, back or muscle pain and visual disturbance are among reported effects. This is not a list of effects that are all common with both medicines. Frequencies differ: Cialis lists dizziness and blurred vision as uncommon, while Viagra lists myalgia as uncommon. Read section 4.8 of the specific product information.

Some people experience troublesome effects even when taking a prescribed dose. Do not assume back pain is simply explained by a longer half-life, that every effect resolves within 48 hours, or that switching medicines yourself is safe. Ask your doctor or pharmacist about persistent or concerning symptoms.

Serious symptoms: act promptly

An erection lasting four hours or more needs immediate medical help because permanent damage can occur.
Sudden vision loss or another sudden visual problem: stop the ED medicine and seek urgent medical care. The labels retain warnings about non-arteritic anterior ischaemic optic neuropathy (NAION); uncertainty in observational studies does not cancel this advice.
Sudden decrease or loss of hearing: stop the medicine and contact a doctor immediately.
Severe dizziness, fainting or chest pain: seek urgent help. Tell emergency staff which ED medicine you took and when. Do not take nitrates to treat chest pain after a PDE5 inhibitor without emergency medical direction. Call 112 for an emergency.

Contraindications and interactions

Nitrates such as glyceryl trinitrate or isosorbide, and nitric oxide donors including recreational “poppers”, must not be combined with either medicine: dangerous hypotension can result. Riociguat is also contraindicated with PDE5 inhibitors. These are not combinations made safe by spacing doses.

Alpha-blockers require product-specific assessment. Tadalafil with doxazosin is not recommended. With sildenafil, patients should be stable on alpha-blocker treatment and the clinician should consider a lower starting dose. “Careful dose spacing” is not a general safety rule. Sildenafil with ritonavir is not recommended; other protease inhibitors and antifungals such as ketoconazole or itraconazole may also affect exposure. Give the clinician a complete list of prescriptions, non-prescription medicines, supplements and recreational substances.

Cardiovascular disease and whether sexual activity is advisable, blood pressure, previous NAION, liver or kidney problems and other conditions require assessment. This is not an exhaustive eligibility checklist. Do not combine ED treatments or alter other medicines yourself. See sections 4.3–4.5 of the Cialis and Viagra product information.

What comparative studies can and cannot tell you

The Salonia et al. 2026 network meta-analysis compared on-demand PDE5 inhibitors across 83 randomised trials with 6,029 participants. Sildenafil 50 mg and 100 mg ranked highly for restoring satisfactory erectile function; tadalafil had lower odds of treatment-related adverse events in the analysis. Those dose-, outcome- and study-specific results do not establish the best medicine for an individual or comparative evidence for daily dosing. Study record, PMID 42296271; publisher abstract and DOI.

Other research uses different methods and populations:

Kim et al. (2022), PMID 35225813: topic modelling of online medication reviews. Such reviews cannot establish which medicine most patients would prefer if offered a choice.
Belladelli et al. (2023), PMID 37861186: claims-database analysis of ocular outcomes. ED/BPH populations and individual endpoints matter.
Etminan et al. (2022), PMID 35389459: a US insurance-database cohort with nested case-control analysis of ocular events. Associations do not establish causation.
Sridharan et al. (2026), PMID 42170200: PAH/FAERS signal analysis. Findings in pulmonary arterial hypertension cannot be assumed to apply to ED treatment or establish the safety of combining ED medicines with nitrates.

These findings do not override current product warnings. Kim, Belladelli, Etminan and Sridharan.

Cost, availability and questions for your doctor

Prices depend on country, pharmacy, presentation, dose and pack size. Brand versus generic prices and current stock must be checked; no universal cheapest-option or EU-wide availability ranking is established here. Check whether any quoted price includes assessment, medicine and delivery, and which services actually apply.

Discuss the frequency of sexual activity, preference about planning, urinary symptoms, previous treatment response and side effects. Tadalafil's longer response window, its daily ED option and Cialis 5 mg's BPH indication may be relevant; sildenafil's shorter action and food-related timing may also matter. Clinical suitability comes before convenience or price. No prescription or result is guaranteed.

Access in Europe and Spain

Prescrivia is a technology intermediary. An independent EU-registered doctor reviews health information and decides whether treatment, more information or referral is needed. Licensed pharmacies are responsible for dispensing; the platform does not prescribe, sell or dispense medicines.

Cialis and Viagra are prescription medicines in the product information used here. Spain prohibits distance sale of prescription medicines. An online consultation or recognition of a cross-border prescription does not establish permission to sell or ship a prescription medicine into Spain. The actual country-specific prescribing, dispensing and any delivery arrangement needs verification; this article makes no legal finding about an uninspected workflow. See BOE, Article 3.5 and AEMPS's patient information.

Sources and review limits

The linked CIMA product information, EMA overview and named studies support the bounded comparison above. They do not constitute approval of this article, a full clinical evidence review or a supply guarantee. See the current clinical, legal and language review statuses for this edition. Discuss your health with a qualified clinician before treatment.

Cialis or Viagra: differences, risks and questions for your doctor