Drug class
PDE5 inhibitor — it slows the enzyme that normally deflates an erection, keeping blood where it belongs during arousal
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Sildenafil guide
What is Sildenafil?
Sildenafil is the active ingredient behind the world's most famous erectile-dysfunction medicines — Viagra and the Kamagra family among them. It was a heart-medicine experiment that stumbled onto something far more famous, and it changed how millions of people talk about and treat erectile dysfunction.
What sildenafil is, in one screen
Sildenafil (sildenafil citrate) belongs to a class of medicines called PDE5 inhibitors. In plain terms, it helps relax blood vessels in the penis so that more blood can flow in during arousal — which makes it easier to get and keep an erection.
It has two established uses. As Viagra, and as the many generic and branded copies that followed, it treats erectile dysfunction (ED). Under the name Revatio, a lower dose treats pulmonary arterial hypertension (PAH), a form of high blood pressure in the lungs.
The crucial detail people miss: sildenafil does not cause an erection by itself. It works in response to sexual stimulation. It is not an aphrodisiac, and it will not make you aroused.
Key facts about sildenafil
The facts that matter before you read any product or brand story.
What it treats
Erectile dysfunction in adults; a lower dose also treats pulmonary arterial hypertension
Needs stimulation
Sildenafil amplifies the body's own arousal response. Without sexual stimulation there is no erection
Onset and window
Usually taken 30–60 minutes before sex; the helpful window is often described as around 4 hours
Strengths
25 mg, 50 mg, and 100 mg tablets are standard. Kamagra products are typically 100 mg
Prescription-only
Sildenafil is a prescription medicine. It is not for everyone, and it interacts with nitrates
The accidental origin
From heart research to the blue pill
Sildenafil is one of medicine's happiest accidents — a heart drug that failed at its first job and succeeded at its side effect.
1989–1991
Synthesised for angina
Pfizer chemists in Kent, England, created a compound (codenamed UK-92,480) to relax blood vessels and treat angina and high blood pressure.
Early 1990s
A memorable side effect
In clinical trials the angina results were modest — but male volunteers kept reporting erections. Pfizer followed the signal instead of discarding it.
1998
FDA approval as Viagra
The US FDA approved sildenafil for erectile dysfunction in March 1998. It became one of the fastest-selling medicines in history.
2005
Second life as Revatio
The same molecule was approved at a lower dose to treat pulmonary arterial hypertension.
2000s onward
Generics and Kamagra
When patents lapsed, affordable generics appeared — including India's Kamagra, now sold worldwide in tablets, jelly, and other forms.
Sildenafil at a glance
Useful orientation, not a promise of individual results.
- First approved
- 1998
- Minutes to onset
- 30–60
- Effect window
- ~4 h
- Standard strengths
- 3
Year the FDA approved sildenafil for erectile dysfunction
Typical guidance for the oral tablet; a heavy meal can delay it
A commonly described window of effect — not a constant erection
25 mg, 50 mg, and 100 mg
Core sources — including Singapore
- NHS: Sildenafil (Viagra)
Plain-language UK summary of what sildenafil is, dosing, side effects, and cautions.
- HealthHub (Singapore): sildenafil medication guide
Singapore's national patient information on sildenafil.
- National Drug Formulary (Singapore): sildenafil
Registered sildenafil products and strengths in Singapore.
- Goldstein I et al. NEJM 1998
The landmark efficacy trial that established oral sildenafil for erectile dysfunction.
Common questions about sildenafil
Is sildenafil the same as Viagra?
Viagra is one brand of sildenafil. Kamagra and many other products also contain sildenafil as their active ingredient. They are not all licensed in the same countries.
Does sildenafil cause an erection on its own?
No. It works only alongside sexual stimulation. It relaxes blood vessels so arousal produces a firmer erection; it does not create arousal.
How long does sildenafil last?
The helpful window is often described as around 4 hours, though this varies. It is not a permanent erection, and an erection lasting more than 4 hours needs urgent medical help.
Can I buy sildenafil over the counter in Singapore?
No. Sildenafil is prescription-only in Singapore. A GP, polyclinic doctor or licensed telehealth service can prescribe it, and a licensed pharmacy dispenses an HSA-registered brand.
Is Kamagra the same strength as Viagra?
Kamagra products are commonly 100 mg of sildenafil per dose, whereas Viagra is also available at 25 mg and 50 mg. Check the label of the exact product.
Can anyone take sildenafil?
No. It is prescription-only and unsuitable for some people, including those taking nitrates or with certain heart conditions. A prescriber must assess suitability.
Now learn how it actually works
Sildenafil's story only makes sense once you understand the chemistry behind an erection. Next, a plain-language walkthrough of the mechanism.