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Sildenafil (Viagra) and tadalafil (Cialis) are both PDE5 inhibitors used to treat erectile dysfunction, but their side effects are not the same: sildenafil more often causes headache, flushing and visual disturbances, while tadalafil is more often linked with back pain and muscle aches, and manufacturer studies have shown a larger mean blood-pressure reduction with sildenafil at the doses studied.
For adults in the UK comparing ED treatment options with a prescriber, those differences can affect which medicine fits best with your health, routine and tolerance for side effects.
This guide compares tadalafil and sildenafil using manufacturer prescribing data, including side effect profiles, blood pressure effects, how long each medicine works and how quickly it starts working, plus food and alcohol interactions, dosage options, serious side effect warnings and the main factors that influence choice between them. Neither medicine is inherently "better" or safer than the other. The right choice depends on an individual clinical assessment, taking into account your health, other medicines and personal circumstances.
How Sildenafil and Tadalafil Work
Both are PDE5 inhibitors. During sexual arousal, the body releases nitric oxide, which relaxes blood vessels in the penis, increasing blood flow. An enzyme called PDE5 normally breaks down the chemical signal responsible for that relaxation. Both medicines block PDE5, helping that signal last longer. Neither creates arousal by itself or increases sexual desire, they work only when you are sexually aroused.
One mechanistic difference worth knowing: tadalafil also inhibits a related enzyme, PDE11, found in skeletal muscle. This is the leading proposed explanation for why back pain and myalgia are reported more often with tadalafil than with sildenafil, which is far more selective for PDE5 over other related enzymes.
Common Side Effects: How the Two Compare
| Side effect | Sildenafil (approx.) | Tadalafil (approx.) |
|---|---|---|
| Headache | ~16% | Reported, generally less frequent than with sildenafil |
| Flushing | ~10% | Reported, generally less frequent than with sildenafil |
| Dyspepsia (indigestion) | ~7%, up to 17% at 100mg | Also reported, broadly comparable |
| Nasal congestion | ~4% | Reported |
| Visual disturbances (colour tinge, light sensitivity) | ~3%, more common at 100mg | Rare |
| Back pain | Uncommon | ~5-6% |
| Myalgia (muscle aches) | Uncommon | ~3-4%, up to ~9% in pooled trial data at higher doses |
Approximate figures from manufacturer clinical-trial data, these figures are not from a single head-to-head trial and should not be interpreted as directly equivalent rates between the two medicines. Both medicines can also cause dizziness and, rarely, priapism (a prolonged erection lasting more than 4 hours), which requires urgent medical attention regardless of which medicine is involved.
Why Tadalafil Causes Back Pain and Sildenafil Does Not (Usually)
This is one of the most search-relevant, genuinely documented differences between the two medicines. In a pooled analysis of 10 placebo-controlled tadalafil trials (1,846 patients), back pain and/or myalgia was reported in 9.4% of patients on tadalafil 10mg and 8.3% on tadalafil 20mg, against 3.7% on placebo. When back pain or myalgia occurs with tadalafil, it may appear several hours after dosing and can last longer than some of sildenafil's common side effects.
Sildenafil, by contrast, does not commonly cause this pattern of muscle or back pain in the same way. Its more characteristic side effects (headache, flushing, visual disturbances) relate more to its vasodilatory action and its comparatively lower selectivity against PDE6, the enzyme found in the retina.
Effect on Blood Pressure: A Genuine, Often-Blurred Difference
This is the point most likely to be oversimplified in generic comparison content, which often quotes a single shared blood-pressure figure for both medicines. Manufacturer data from studies in healthy subjects shows a larger mean blood-pressure reduction with sildenafil at the doses studied.
| Medicine | Mean maximum supine BP decrease vs placebo (single dose, healthy subjects) |
|---|---|
| Sildenafil 100mg | 8.4/5.5 mmHg |
| Tadalafil 20mg | 1.6/0.8 mmHg |
Sildenafil's effect on blood pressure is considerably larger than tadalafil's in this specific comparison. This does not mean tadalafil carries no cardiovascular caution, and it does not remove the need for individual assessment, both sildenafil and tadalafil carry the same absolute contraindication with nitrate medicines, nicorandil, riociguat and recreational nitrates (poppers), since combining any PDE5 inhibitor with these can cause a sudden drop in blood pressure.
Nitrate medications and alpha-blockers also require medical supervision, and both medicines can interact with other medicines, so you should disclose all prescription medicines, over-the-counter medicines and supplements to your prescriber or pharmacist.
But if a shared, unattributed "8-10mmHg for both" figure has come up in other comparison content you have read, it is not an accurate representation of the manufacturer data for tadalafil specifically.
Duration: Why Tadalafil's Side Effects Can Last Longer
Sildenafil can support erectile function for several hours after dosing, while tadalafil can remain effective for up to 36 hours. Tadalafil can remain active for up to 36 hours after taking tadalafil, food-independent, which is why it is sometimes described informally as the "weekend pill".
This longer window is often seen as an advantage for spontaneity, because tadalafil remains in the body for longer, some side effects may also persist for longer. In practice, sildenafil may suit people who prefer to plan sexual activity, while some people may prefer tadalafil for its longer window of effect.
Onset: Which Works Faster
Sildenafil's median time to peak plasma levels is around 60 minutes, and sildenafil typically starts working within 30 to 60 minutes, though the documented range is roughly 30 to 120 minutes depending on the individual and whether it is taken with food. It should generally be taken about one hour before sexual activity.
Tadalafil takes somewhat longer to reach peak levels for most people, though because it remains active for up to 36 hours, exact timing before sexual activity matters less than with sildenafil. Neither difference means one medicine is faster-acting in every individual case, response varies.
Food and Alcohol
A high-fat meal delays sildenafil's absorption, with manufacturer data showing a mean 60-minute delay in time to peak concentration and a 29% reduction in peak concentration, though total absorption is not significantly affected.
Taking sildenafil with a high-fat meal can delay its onset, so it is often taken without a large or high-fat meal when a quicker onset is preferred.
Tadalafil's absorption is not affected by food, which is one of its more consistently documented practical advantages.
Regarding alcohol, both are PDE5 inhibitors and share the same general caution: excessive alcohol can independently affect the ability to get or keep an erection and can contribute to dizziness or low blood pressure. Manufacturer data for tadalafil specifically found that a moderate amount of alcohol (around 4 standard drinks) did not meaningfully worsen its blood-pressure effects, while larger amounts increased the risk of dizziness and low blood pressure. Neither medicine has an officially stated "safe unit" figure specific to combining it with alcohol.
Dosage
| Medicine | Strengths available in the UK | Typical use pattern |
|---|---|---|
| Sildenafil / Viagra | 25mg, 50mg, 100mg | Taken as needed before sexual activity, with 50mg as the typical starting dose, one dose per 24 hours |
| Tadalafil / Cialis | 10mg, 20mg (on-demand), 2.5mg, 5mg (daily use) | Available as an as-needed dose or, separately, as tadalafil daily in a low dose option |
| Cialis Daily / Cialis Together | 5mg (daily) / 10mg (on-demand, P-medicine) | Cialis Together is available following a pharmacist consultation without a prescription |
The specific strength and dosing pattern appropriate for you is determined during your clinical assessment, taking into account your health, other medicines and previous response to treatment. This is a decision made with your prescriber, not a general recommendation.
Serious Side Effects: The Same Warnings Apply to Both
- Priapism, a prolonged and potentially painful erection lasting more than 4 hours, is a medical emergency requiring urgent attention regardless of which medicine is involved.
- Sudden decrease or loss of vision or hearing has rarely been reported with PDE5 inhibitors generally. Stop taking the medicine and seek medical advice promptly if this occurs.
- Severe allergic reactions are rare but require immediate medical attention, including swelling of the face, lips, tongue or throat, or difficulty breathing.
Choosing Between Them: Questions Worth Raising With Your Prescriber
- Are you choosing an ed treatment or ed medication based on lifestyle factors and health conditions, such as side effects, timing, and other medicines you take?
- Do you have a history of headaches or visual sensitivity, which are more associated with sildenafil?
- Do you do physical exercise or activity where back pain or muscle aches, more associated with tadalafil, would be disruptive?
- How much advance planning do you typically have? Tadalafil's longer window may suit less-planned activity better than sildenafil's shorter one, and daily low-dose tadalafil may suit frequent sexual activity better than on-demand sildenafil.
- Do you take other blood pressure medication, where sildenafil's larger blood-pressure effect may be more relevant to discuss?
- Is cost a significant factor, given the price difference between generic sildenafil and tadalafil-based treatments? Personal preference and desired erection quality can also affect which option feels more suitable.
None of these questions has a universally correct answer. They are the practical considerations a prescriber weighs alongside your medical history during assessment, with the aim of improving sexual function and helping you achieve a satisfactory erection, not finding one universally better medicine.
Medical Disclaimer
This guide is provided for general information only and does not constitute medical advice. It is not a substitute for consultation with a qualified healthcare professional. Sildenafil and tadalafil are medicines used to treat erectile dysfunction, with sildenafil citrate and tadalafil as the active ingredients in Viagra and Cialis, respectively. They should be used following appropriate assessment and advice from a qualified healthcare professional. This article does not state that either medicine is better than the other or provide dosing instructions, suitability depends on an individual clinical assessment, and results and side effects can vary between individuals. Do not start, stop or switch between these medicines without speaking to your prescriber or pharmacist first. If you experience an erection lasting 4 hours or longer, chest pain, or symptoms of a severe allergic reaction, seek urgent medical attention.
