Sildenafil BNF Guidelines: What to Avoid

Understanding sildenafil contraindications and warnings is essential for safe treatment. The British National Formulary (BNF) outlines specific medications, conditions, and circumstances that require caution or complete avoidance when taking sildenafil. This guidance applies to both generic sildenafil and branded Viagra, helping patients make informed decisions about their ED treatment options.

  • Never combine sildenafil with nitrate medications (GTN spray, isosorbide)
  • Avoid if you have severe heart disease or recent cardiovascular events
  • Check interactions with alpha-blockers and blood pressure medications
  • Limit alcohol consumption as it may increase side effects
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Sildenafil
~60 min
~4 hrs
you like to plan ahead
Tadalafil (as-needed)Most prescribed
~30 min
up to 36 hrs
you want a whole weekend covered
Tadalafil Daily
always active
continuous
you never want to think about timing
Vardenafil
~15–30 min
~8 hrs
you want the fastest onset

How ED treatment works in your body.

Increases blood flow

Increases blood flow

When PDE5 is suppressed, blood is able to flow more freely through the body—including to the (you guessed it) penis.

Increase erections

Increase erections

More blood flow to the penis means harder, more satisfying erections once you're sexually stimulated.

Complete BNF Contraindications and Warnings

Dangerous Drug Combinations to Avoid

The BNF clearly states that sildenafil must never be taken with nitrate medications including GTN spray, isosorbide mononitrate, or isosorbide dinitrate. This combination can cause a dangerous drop in blood pressure that may be life-threatening. Riociguat, used for pulmonary hypertension, is also contraindicated with sildenafil treatment.

Cardiovascular Contraindications

According to BNF guidelines, sildenafil should be avoided in men with severe heart disease, recent heart attack or stroke within six months, or very low blood pressure. Those with unstable angina or severe heart failure should not use sildenafil. A cardiovascular assessment is essential before starting treatment.

Medication Interactions Requiring Caution

Alpha-blockers used for prostate problems or blood pressure may interact with sildenafil, potentially causing dizziness or fainting. The BNF recommends starting with lower doses if these medications cannot be avoided. Strong CYP3A4 inhibitors like ketoconazole or ritonavir may increase sildenafil levels, requiring dose adjustment.

Medical Conditions Requiring Careful Assessment

Men with active peptic ulcers, bleeding disorders, or anatomical penile abnormalities need specialist evaluation before sildenafil use. Those with severe liver or kidney impairment may require dose reduction or alternative treatments. Vision problems, particularly NAION (non-arteritic anterior ischaemic optic neuropathy), warrant immediate medical attention.

Lifestyle Factors to Consider

The BNF advises limiting alcohol consumption as it may worsen erectile function and increase side effects like dizziness or headaches. Grapefruit juice should be avoided as it can affect how sildenafil is metabolised. Patients can choose between branded Viagra or generic sildenafil at EverydayMeds, with both options following the same BNF safety guidelines.

What you need to know about erectile dysfunction

It's one of the most common conditions we treat, and one of the most treatable.

What is Erectile Dysfunction?
ED means finding it hard to get or keep an erection firm enough for sex. It's not unusual to have an off night; the point at which it's worth doing something is when it's happening regularly, or when it's starting to bother you.
How will I feel when I take ED medication?
For most men, nothing feels different day to day. The medication works in the background — when you're sexually aroused, it helps blood flow where it's needed, so erections come more easily and last longer. It doesn't cause arousal on its own. Some men notice mild, short-lived effects like a headache, facial flushing or a stuffy nose, which usually pass as the dose wears off.
Why do I need a prescription for ED medication?
ED treatments are prescription-only medicines in the UK because they aren't suitable for everyone — for example, alongside certain heart conditions or medications like nitrates. That's why we ask you to complete a short online consultation first. A UK-registered prescriber reviews your answers to make sure your chosen treatment is safe and right for you before it's dispensed.
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