- Sildenafil (brand name Viagra) is taken about an hour before sex and works for roughly 4–5 hours.
- Tadalafil (brand name Cialis) can work for up to 36 hours, and can also be taken as a low daily dose.
- Both need sexual arousal to work. Neither is safe with nitrates.
How they work
Sildenafil and tadalafil belong to the same drug family, called PDE5 inhibitors. During arousal, the body releases signals that relax the blood vessels in the penis so blood can flow in. PDE5 is an enzyme that switches that signal off. By blocking it, these medicines help the signal last longer, making it easier to get and keep an erection.
Important: they don't cause an erection on their own. Without arousal, nothing happens.
Side-by-side comparison
| Sildenafil | Tadalafil | |
|---|---|---|
| Common brand | Viagra | Cialis |
| Starts working | About 30–60 minutes | About 30 minutes to 2 hours |
| Lasts | About 4–5 hours | Up to 36 hours |
| Usual doses | 25, 50 or 100 mg as needed | 10 or 20 mg as needed, or 2.5–5 mg daily |
| Effect of a heavy, fatty meal | Can delay it noticeably | Little effect |
| Typical side effects | Headache, flushing, stuffy nose, indigestion, short-lived blue-tinted vision | Headache, flushing, stuffy nose, indigestion, back or muscle aches |
Which one suits which situation?
Sildenafil may suit you if…
- You have sex occasionally and can plan an hour ahead.
- You prefer a medicine that clears your system faster.
- Cost matters: generic sildenafil is widely available and usually inexpensive.
Tadalafil may suit you if…
- You want spontaneity, without timing sex around a pill.
- You have sex several times a week, where a daily low dose can make sense.
- You also have urinary symptoms from an enlarged prostate (BPH); daily tadalafil is approved for both.
Many men try both before settling on one. A doctor can also adjust the dose if the first one doesn't work well. Not responding to the first try doesn't mean the medicine won't work for you: it is often a matter of dose, timing or taking it with a big meal.
Alcohol
A drink or two is generally fine for most people, but heavy drinking makes erections harder on its own and increases the chance of dizziness and low blood pressure with either medicine.
Who should not take them
Talk to a doctor first if you have heart disease, have recently had a heart attack or stroke, have very low or uncontrolled high blood pressure, have serious liver or kidney problems, or take alpha-blockers or certain HIV or antifungal medicines.
Get emergency help if you have an erection lasting more than 4 hours, chest pain, or sudden loss of vision or hearing.
Avoid "herbal Viagra"
Products sold online or in shops as natural erection boosters are a known risk. Regulators repeatedly find them secretly contaminated with sildenafil, tadalafil or similar drugs, in unknown doses and without warnings. Get ED medication from a licensed pharmacy or clinic instead.
Getting a prescription
Depending on where you live, ED medication is available through your own doctor, from a pharmacist, or via licensed online clinics after a medical questionnaire. Whichever route you choose, answer health questions honestly: the safety checks are there for a reason.
Sources
- U.S. FDA prescribing information: Viagra (sildenafil citrate) and Cialis (tadalafil).
- NHS: Sildenafil and Tadalafil medicine guides.
- American Urological Association: Erectile Dysfunction Guideline (2018, amended 2023).
- U.S. FDA: Tainted sexual enhancement products, public notifications.