Viagra side effects: A complete guide for UK patients

DoktorABC editorial team

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

Why does Viagra cause side effects?

Viagra works by inhibiting PDE5, an enzyme that breaks down cyclic GMP in blood vessel walls. This produces vasodilation (widening of blood vessels) not only in the penile arteries where it is therapeutically intended, but to a lesser degree throughout the body. Most of Viagra's common side effects are a direct consequence of this systemic vasodilatory effect rather than a toxic reaction.

Most common Viagra side effects

The frequency data below is drawn from the Viagra Summary of Product Characteristics at medicines.org.uk, based on clinical trial data reviewed by the MHRA.

Very common side effects (affecting more than 1 in 10 people)

• Headache: The most frequently reported side effect, caused by vasodilation in cranial blood vessels. 

Common side effects (affecting 1-10 in 100 people)

• Facial flushing: Redness and warmth in the face, caused by increased blood flow
• Indigestion and acid reflux: Sildenafil relaxes smooth muscle in the lower oesophageal sphincter, allowing stomach acid to reflux
• Nasal congestion: Vasodilation in nasal mucosa causes a blocked or runny nose
• Dizziness: Related to the blood pressure-lowering effect of sildenafil
• Visual disturbances: Including a bluish tinge to vision, increased light sensitivity, or blurred vision. More common at higher doses and related to sildenafil's partial inhibition of PDE6 in the retina
• Nausea

Most common side effects are mild to moderate, temporary, and resolve as the medication is metabolised. They are more pronounced at the 100 mg dose.

Is Viagra safe?

Yes, for most men who have been appropriately assessed and have no contraindications. The MHRA has licensed sildenafil for ED in the UK, and it has been prescribed to hundreds of millions of men worldwide since 1998.

Viagra is not safe for all men. It is absolutely contraindicated with nitrate medications, and its use requires caution in men with cardiovascular disease, low blood pressure, severe liver or kidney impairment, and certain eye conditions. A clinical assessment before starting treatment is not a formality; it is a necessary safety step.

Long-term side effects of Viagra: What does the evidence show?

Sildenafil has been in clinical use since 1998, providing over two decades of post-marketing safety data. Long-term studies have not identified new serious safety signals beyond those known from the original trials.

The available evidence does not suggest that long-term use at recommended Viagra doses causes cumulative organ damage, dependency, or tolerance (a reduction in effectiveness over time). Men who take sildenafil regularly for years typically report a consistent response, provided underlying ED causes are not progressing.

Ongoing monitoring through the MHRA Yellow Card scheme continues to capture post-market safety data. The current evidence supports sildenafil as a safe long-term option for eligible men under appropriate prescriber oversight.

Side effects of Viagra in older men

Older men are more sensitive to sildenafil for two reasons. First, reduced kidney and liver function in men over 65 means the drug is cleared more slowly, producing higher blood concentrations from the same dose. Second, older men are more likely to have underlying cardiovascular conditions and to be taking medications that interact with sildenafil.

The Viagra Summary of Product Characteristics recommends a starting dose of 25 mg for men over 65, with titration to 50 mg or 100 mg only if the lower dose is well tolerated and insufficient. The most clinically significant concern in older men is hypotension (low blood pressure), particularly in those also taking antihypertensive medications.

Can Viagra cause a heart attack or stroke?

Viagra does not directly cause heart attacks or strokes in healthy men. However, sexual activity itself places cardiovascular demands on the body, and for men with pre-existing severe cardiovascular disease, this exertion carries risk regardless of medication.

Sildenafil causes a modest reduction in blood pressure. In men with stable cardiovascular disease who are not taking nitrates, this is generally well tolerated. The concern arises in men with unstable angina, recent myocardial infarction or stroke (within the past six months), or severely impaired cardiac function. In these men, the combination of sildenafil and the cardiovascular demands of sexual activity may pose an unacceptable risk. Alcohol compounds sildenafil's blood pressure-lowering effect and can significantly worsen side effects including dizziness and flushing. See our guide to Viagra and alcohol for more detail on managing this combination safely. 

The absolute contraindication with nitrates is separate and more serious. Nitrates and sildenafil both lower blood pressure through different mechanisms, and their combination can produce a severe, potentially life-threatening drop in blood pressure.

Can Viagra cause acid reflux?

Yes. Indigestion and acid reflux are listed as common side effects in the Viagra SmPC. Sildenafil relaxes smooth muscle throughout the body, including the lower oesophageal sphincter, the valve that prevents stomach acid from rising into the oesophagus. This can allow acid to reflux, causing heartburn or indigestion.

This effect is more likely at higher doses and when Viagra is taken close to a large meal. Taking Viagra on a light stomach and remaining upright for at least 30 minutes after taking it can reduce discomfort.

Why does Viagra cause headaches?

Headache is the most commonly reported side effect of sildenafil. The mechanism is the same vasodilation that produces the therapeutic effect: as blood vessels throughout the body dilate, including those supplying the brain, intracranial pressure changes can trigger tension-type or vascular headaches.

Headaches are more common at higher doses and typically resolve within a few hours as the medication is metabolised. Staying well hydrated before and after taking Viagra reduces the likelihood and severity of headaches. Standard over-the-counter pain relief such as paracetamol is generally effective if headaches occur.

Can Viagra cause depression?

There is no established causal link between sildenafil and depression. Clinical trials have not shown a higher rate of depression in men taking Viagra compared to placebo.

The relationship between ED and mental health is, however, well established. ED is strongly associated with anxiety, depression, and reduced self-esteem, and these psychological effects can persist even when erectile function is improved by medication. For men whose ED has caused significant psychological impact, it is clinically appropriate to address both psychological and physical symptoms.

If you experience low mood, anxiety, or other mental health symptoms while taking Viagra, speak to your GP. The cause is more likely to be the ED itself or an underlying condition, rather than a direct effect of the medication.

Does Viagra cause hair growth?

No. There is no clinical evidence that sildenafil causes hair growth. This claim appears to originate from very early laboratory research on sildenafil's effects on nitric oxide pathways in hair follicles, but it has not been confirmed in clinical trials and is not listed as an effect in the prescribing information. Sildenafil is not used or licensed for hair growth in the UK.

Serious and rare side effects

While rare, the following side effects require immediate medical attention.

Priapism

A prolonged erection lasting more than four hours that is not associated with sexual stimulation. Priapism is a medical emergency. If left untreated, it can cause permanent damage to penile tissue. If you experience an erection lasting more than four hours, go to A&E immediately.

Sudden vision loss

Non-arteritic anterior ischaemic optic neuropathy (NAION), a sudden reduction in blood flow to the optic nerve causing vision loss in one eye, has been reported in very rare cases. The MHRA updated sildenafil prescribing information in February 2026 to highlight this risk, estimated to affect up to 1 in 10,000 people. If you notice sudden vision loss or changes in one eye, attend eye casualty or A&E immediately.

Sudden hearing loss

Sudden decrease or loss of hearing has been reported in rare cases. If this occurs, stop taking Viagra and seek medical attention promptly.

Severe allergic reaction

Signs include swelling of the face, lips, tongue, or throat, difficulty breathing, or severe skin reactions. Seek emergency medical help immediately.

Who should not take Viagra?

Viagra must not be used by men who:

• Take nitrate medications in any form (including glyceryl trinitrate, isosorbide mononitrate, or amyl nitrite)
• Have been advised by a doctor to avoid sexual activity due to cardiovascular risk
• Have had a recent stroke or heart attack (within the past six months)
• Have severe liver impairment
• Have low blood pressure (hypotension) or uncontrolled high blood pressure
• Have a history of non-arteritic anterior ischaemic optic neuropathy (NAION)
• Have a known hypersensitivity to sildenafil or any excipient in the formulation

Viagra should be used with caution in men with anatomical deformity of the penis, conditions predisposing to priapism (such as sickle cell anaemia), or retinitis pigmentosa.

Final thoughts

Viagra has a well-characterised safety profile and is appropriately prescribed to millions of men each year. For most men without contraindications, common side effects are mild, temporary, and manageable. Serious side effects are rare but require prompt attention. The most important safety principle is straightforward: ensure a proper clinical assessment before starting treatment, disclose all current medications to your prescriber, and do not take Viagra if you are on nitrates. For an overview of how men rate their experience of Viagra in practice, see our Viagra reviews guide. 

DoktorABC connects UK patients with licensed prescribers who can assess suitability for treatment and provide ongoing clinical guidance.

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The DoktorABC medical advisory board

Dr. Roland Ruiken

Medical advisor, Norway

Dr. Viktor Simunovic

Medical advisor, Croatia

To the medical advisory board