> ## Documentation Index
> Fetch the complete documentation index at: https://docs.hi-doctor.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Viagra (sildenafil)

> How sildenafil (Viagra) works for erectile dysfunction, EU-authorised since 1998: mechanism, dosing, side effects and the nitrate interaction.

Sildenafil is a prescription tablet for erectile dysfunction, first authorised across the EU in 1998. It relaxes the blood vessels in the penis so that more blood can flow in when you are sexually aroused. It does not create arousal on its own — sexual stimulation is still needed for it to work.

<Info>
  **Active ingredient** sildenafil  ·  **Dosages** 25 / 50 / 100 mg on demand  ·  **Authorised in the EU** since 13 September 1998
</Info>

## What it is

Viagra is centrally authorised, so a single EU authorisation covers all 27 member states, held by Upjohn EESV in the Netherlands. Alongside the film-coated tablets, the authorisation includes a 50 mg orodispersible tablet that dissolves in the mouth. It is indicated for erectile dysfunction in men — the inability to achieve or maintain an erection sufficient for satisfactory sexual performance.

## How it works

<CardGroup cols={3}>
  <Card title="Arousal releases nitric oxide" icon="bolt">
    During sexual stimulation, nerves and blood vessel linings in the penis release a gas called nitric oxide. Nitric oxide switches on an enzyme, guanylate cyclase, which makes a messenger chemical called cyclic GMP inside the erectile tissue.
  </Card>

  <Card title="Cyclic GMP lets blood in" icon="droplet">
    Cyclic GMP relaxes the smooth muscle in the corpus cavernosum, the two spongy columns that run the length of the penis. As that muscle relaxes, blood flows in faster than it drains away, and the penis becomes firm.
  </Card>

  <Card title="Sildenafil blocks the PDE5 enzyme" icon="lock">
    Sildenafil is a phosphodiesterase-5 (PDE5) inhibitor. It relaxes smooth muscle in the penile blood vessels, facilitating an erection in response to sexual stimulation — it does not cause spontaneous erections or change desire.
  </Card>
</CardGroup>

<CardGroup cols={3}>
  <Card title="Works on demand" icon="clock">
    Onset in 30–60 minutes; effect window of \~4–6 hours.
  </Card>

  <Card title="Adjustable dose" icon="sliders">
    25 / 50 / 100 mg — the doctor selects the dose that fits your case.
  </Card>

  <Card title="Doctor-screened" icon="user-doctor">
    The best-known, most-studied PDE5 inhibitor.
  </Card>
</CardGroup>

## What the studies reported

The pivotal trial (*N Engl J Med* 1998, "Oral sildenafil in the treatment of erectile dysfunction", PMID [9580646](https://pubmed.ncbi.nlm.nih.gov/9580646/)) and a dedicated colour-vision study underpin the EU product information. Both studies were short (12 and 24 weeks) and relied on self-reported questionnaires and diaries, so they describe short-term effect rather than long-term outcomes. Full detail is in the EMA EPAR (EMEA/H/C/000202) and its Summary of Product Characteristics, sections 4.2, 4.8 and 5.1.

## What has been reported

<Accordion title="Side effects, from the EU product information">
  | Effect                                                                                       | Frequency                        |
  | -------------------------------------------------------------------------------------------- | -------------------------------- |
  | Headache                                                                                     | Very common (at least 1 in 10)   |
  | Flushing                                                                                     | Common (1 in 100 to 1 in 10)     |
  | Dyspepsia (indigestion)                                                                      | Common (1 in 100 to 1 in 10)     |
  | Nasal congestion                                                                             | Common (1 in 100 to 1 in 10)     |
  | Visual disturbance / colour distortion / blurred vision                                      | Common (1 in 100 to 1 in 10)     |
  | Non-arteritic anterior ischaemic optic neuropathy (NAION) — sudden loss of vision in one eye | Rare (1 in 10,000 to 1 in 1,000) |
</Accordion>

In the pivotal trial, headache, flushing and dyspepsia were the most common effects, occurring in 6% to 18% of men. A dedicated study found mild, temporary blue/green colour-discrimination changes 1 hour after a 100 mg dose, with no effect remaining 2 hours after the dose. Sildenafil is contraindicated in men who have already lost vision in one eye from NAION.

<Warning>
  Contraindicated with nitrate medicines and nitric-oxide donors — including recreational "poppers" — because the combination can cause a serious drop in blood pressure.
</Warning>

## How to take it

<CardGroup cols={2}>
  <Card title="Timing" icon="clock">
    Take it 30–60 minutes before sexual activity.
  </Card>

  <Card title="Food" icon="utensils">
    Avoid heavy, high-fat meals right before — they delay onset.
  </Card>

  <Card title="Nitrates" icon="triangle-exclamation">
    Never combine with nitrates.
  </Card>

  <Card title="Adjustment" icon="sliders">
    Message your doctor to adjust the dose if needed — never more than one dose in 24 hours.
  </Card>
</CardGroup>

## How Hi-Doctor handles it

An EU-licensed doctor reviews your case and, if eligible, issues an electronic prescription valid at any EU pharmacy under Directive 2011/24/EU. See [pricing](/essentials/pricing) for the consultation fee — the medicine is paid separately at a pharmacy. For a longer-acting PDE5 option, see [tadalafil](/library/medicines/tadalafil), and for the eligibility checks that apply, see [safety](/essentials/safety).

## Start a consultation

<Card title="Sexual health consultation" icon="stethoscope" href="https://hi-doctor.ai/consultation/sexual-health/1">
  Answer the sexual health questionnaire and a registered doctor reviews it.
  They decide whether treatment is appropriate, which one, and at what dose —
  and declining is a normal outcome. See [what it costs](/essentials/pricing)
  and [how the review works](/essentials/how-it-works).
</Card>

<Warning>
  This page is reference information, not medical advice and not a prescription.
  Whether a medicine is appropriate for you is decided by a registered doctor
  after reviewing your consultation.
</Warning>