← All medications

Sildenafil

Medication PDE5 inhibitor other
Aka: Viagra
Brands seen in Singapore: Viagra, Revatio, Sildegra, Olvion, Maxigra
Not medical advice. Check with your doctor or pharmacist before you start, stop or change the timing of anything you take.
How to use
What it’s for
Erectile dysfunction; also pulmonary arterial hypertension
When to take
As-needed, about 1 hour before sex; once a day at most
Take with
Water · with or without food; a fatty meal slows onset
Pros & cons
Pros
  • HealthHub: works within 1 hour, lasts up to 4 hours
  • HealthHub: may be taken with or without food
  • SG label: 25, 50 and 100 mg strengths registered here
Cons
  • HealthHub: flushing, headache, blue-tinged vision
  • SG label: nitrates are contraindicated with it
  • HealthHub: an erection over 4 hours is an emergency
Sources
  1. HSA — Listing of Registered Therapeutic Products (data.gov.sg open dataset, 5,511 rows, cached in full and filtered locally) · read 2026-08-27
    what it says
    “All 23 registered sildenafil products carry Forensicclassification 'Prescription Only' — SIN10804P VIAGRA TABLET 25 mg · SIN10805P VIAGRA TABLET 50 mg · SIN10806P VIAGRA TABLET 100 mg · SIN13244P Revatio Film-Coated Tablets 20mg · SIN14188P Sildegra Tablet 100mg”
  2. Singapore National Drug Formulary — VIAGRA TABLET 100 MG [SIN10806P], section 4.1 therapeutic indications · read 2026-08-27
    what it says
    “Sildenafil is indicated for the treatment of erectile dysfunction, which is the inability to achieve or maintain a penile erection sufficient for satisfactory sexual performance. In order for sildenafil to be effective, sexual stimulation is required.”
  3. Singapore National Drug Formulary — VIAGRA TABLET 100 MG [SIN10806P], section 4.2 posology · read 2026-08-27
    what it says
    “For most patients, the recommended dose is 50 mg taken, as needed approximately 1 hour before sexual activity … The maximum recommended daily dose is 100 mg. The maximum recommended dosing frequency is once per day.”
  4. Singapore National Drug Formulary — VIAGRA TABLET 100 MG [SIN10806P], use with other medications · read 2026-08-27
    what it says
    “If ritonavir is co-administered with sildenafil, it is recommended not to exceed a maximum single dose of 25 mg of sildenafil in a 48-hour period.”
  5. Singapore National Drug Formulary — VIAGRA TABLET 100 MG [SIN10806P], section 4.3 contraindications · read 2026-08-27
    what it says
    “administration to patients who are concurrently using nitric oxide donors, organic nitrates or organic nitrites in any form either regularly or intermittently is therefore contraindicated”
  6. HealthHub (National Medication Information workgroup, SG) — Sildenafil: what is this medication for · read 2026-08-27
    what it says
    “Sildenafil is used to treat erectile dysfunction, a condition where you are unable to get or keep an erection firm enough to have sexual intercourse … Therefore it is taken on an as-needed basis.”
  7. HealthHub — Sildenafil: how should I take this medication · read 2026-08-27
    what it says
    “It starts to work within 1 hour and is effective up to 4 hours before sexual activity … takes longer to start working if you take it with a high-fat meal”
  8. HealthHub — Sildenafil: taking it with food · read 2026-08-27
    what it says
    “This medication may be taken with or without food.”
  9. HealthHub — Sildenafil: common side effects · read 2026-08-27
    what it says
    “Facial flushing where the skin on your face feels hot · Headache · Giddiness due to lowered blood pressure · Visual disturbances (including blurring of vision, blue tinge in vision) · Indigestion or stomach pain · Nosebleed”
  10. HealthHub — Sildenafil: rare but serious side effects · read 2026-08-27
    what it says
    “Priapism (an erection that will not go away) - If you have an erection that lasts more than 4 hours, get medical attention immediately at the emergency department … Sudden vision loss in one or both eyes”
  11. HealthHub — Sildenafil: what precautions should I take · read 2026-08-27
    what it says
    “The combination of nitrates with sildenafil may lead to a dangerous fall in your blood pressure. Nitrates must be withheld at least 24 hours after the last dose of Sildenafil … Alpha-1 Blockers (such as Tamsulosin, Terazosin, Prazosin, Alfuzosin)”
  12. HealthHub — Sildenafil: what food or medication should I avoid · read 2026-08-27
    what it says
    “Avoid taking grapefruit, or grapefruit juice as they may interact with your medication and increase the risk of side-effects.”
Report a correction View as JSON

Updated 2026-08-29. Every line above traces to a numbered source. Kozen-X is not a medical device and does not give medical advice. It lists what published sources have written down — nothing more. A pair not listed is unrecorded, not cleared. Before you start, stop or re-time anything, talk to your doctor or pharmacist.