Viagra (Sildenafil): How It Works, Doses on the Label and Safety
Sildenafil has helped millions of men since 1998. It works well for most, but only with arousal, and it can be dangerous with one group of heart medicines.
SteroidsCanada.org editorial teamUpdated · 3 min read
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Sildenafil was originally studied as a heart medicine. Researchers noticed an unexpected side effect in trial volunteers, and in 1998 it launched as Viagra, the first pill for erectile dysfunction. It remains one of the most prescribed medicines in the world.
How it works
An erection depends on blood flowing into the penis and staying there. Arousal releases nitric oxide, which raises a messenger called cGMP that relaxes blood vessel walls. The enzyme PDE5 breaks cGMP down. Sildenafil blocks PDE5, so the signal lasts longer and the response to arousal is stronger. Without arousal, nothing happens.
Label dosing
The label starts most men at 50 mg about an hour before sex, no more than once a day, adjusting to 25 mg or 100 mg depending on effect and side effects. Men over 65, or with liver or kidney problems, often start at 25 mg.
Side effects
- Headache and facial flushing (the most common)
- Stuffy nose and indigestion
- Bluish tint or light sensitivity in vision, briefly
- Dizziness when standing up quickly
Medicines that interact
Alpha-blockers for prostate or blood pressure, some HIV medicines, and strong antifungals or antibiotics such as ketoconazole and clarithromycin can raise sildenafil levels or add to its blood pressure effect. A pharmacist can check your list in a few minutes.
When erection problems are a warning sign
Erectile dysfunction can be an early sign of heart disease, diabetes or low testosterone, because the blood vessels involved are small and show damage first. A check-up with blood pressure, blood sugar and cholesterol is worthwhile, especially if problems start before 50.
Getting the best result
- Take it about an hour before sex, on a light stomach.
- Give it a fair trial: doctors usually suggest trying a dose on several occasions before deciding it does not work.
- Remember that arousal is still needed; it will not create an erection on its own.
- Limit alcohol, which works against erections.
- Do not take more than one dose in 24 hours.
Who should not take sildenafil
- Anyone using nitrates or riociguat
- People told by a doctor that sexual activity is unsafe for their heart
- Anyone who has had a recent heart attack or stroke, or has very low blood pressure
- People with certain inherited eye conditions such as retinitis pigmentosa
- Anyone who has had sudden vision loss in one eye (NAION)
Sildenafil for other conditions
Under a different brand, sildenafil is approved to treat pulmonary arterial hypertension, at smaller doses taken three times a day. That is a specialist treatment. It also explains why the drug lowers blood pressure slightly, which is usually harmless in healthy men but matters with heart medicines.
Myths and facts
| Myth | Fact |
|---|---|
| Viagra increases desire | It improves blood flow in response to arousal; it does not change libido. |
| A higher dose always works better | Above 100 mg, side effects rise without much extra benefit. |
| It causes heart attacks | In men cleared for sexual activity, large studies have not found higher cardiac risk; the danger is with nitrates. |
Sildenafil compared with tadalafil
Sildenafil
- Works in 30 to 60 minutes
- Lasts 4 to 6 hours
- Slowed by fatty meals
Tadalafil
- Works in 30 minutes to 2 hours
- Lasts up to 36 hours
- Less affected by food
Product pages: Zionova Viagra, Genetix Pharma Viagra and Zionova Cialis.
Products in this guide
Each product page carries the chemistry, label strength, risks and references.
Questions
How long does sildenafil last?
It works for about four to six hours, not as a constant erection but as an improved response to arousal during that window.
Does food affect it?
A heavy, fatty meal can delay the effect by up to an hour. Taking it on a lighter stomach helps it work sooner.
Is it safe to drink alcohol with it?
Moderate drinking is usually fine, but alcohol lowers blood pressure and blunts erections, which works against the drug.
Is sildenafil a prescription drug in Canada?
Yes. Sildenafil is on Canada's Prescription Drug List; a doctor or pharmacist can assess whether it suits you.
How is it different from Cialis?
Tadalafil (Cialis) lasts up to 36 hours and is less affected by food. Sildenafil acts faster and wears off sooner.
Sources
General information, not medical advice. Written for adults in Canada. Anabolic steroids are Schedule IV under the Controlled Drugs and Substances Act. Talk to a licensed healthcare provider and get bloodwork before using any compound.
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