What are these two medicines?
Sildenafil and tadalafil belong to the same class, PDE5 inhibitors, which the European Association of Urology (EAU) recommends as first-line medical treatment for erectile dysfunction (ED).[1] Many people know them by famous brand names; this article uses the generic names for accuracy.
A common misunderstanding is that these medicines create desire or a constant erection. They do neither. They do not increase libido, and they do not cause an erection without stimulation. They help the body's natural response to sexual stimulation work better.[1],[2]
Why choosing the right one matters
Overall, the two medicines work about equally well; neither is “stronger” for everyone. What differs is how they act over time, which matters a great deal in real life. Some men want flexibility without planning around a dose; others have sex occasionally and prefer a medicine that wears off sooner. Matching the medicine to your lifestyle, health and current medicines improves both results and safety.[1]
Safety matters just as much. Both medicines can interact dangerously with certain drugs and cause a severe drop in blood pressure, so buying them without assessment carries more risk than many people realise.
How widely are they used?
PDE5 inhibitors are the most widely used ED treatments worldwide, but that does not mean every man with ED needs one or can safely take one. Suitability depends on medical conditions, current medicines and personal circumstances.[1] Rules on supply differ between countries; in Thailand they are dispensed by a doctor or pharmacist.
Same mechanism, different timing
During sexual stimulation the body produces cGMP, which relaxes the smooth muscle in penile blood vessels so blood can flow in and create an erection. The PDE5 enzyme breaks cGMP down. Both medicines block this enzyme, so erections occur more easily and last longer.[1],[2]
The key difference is how long each medicine stays active. According to the EAU[1]:
- Sildenafil starts working about 30–60 minutes after a dose, and its window of effect can last up to about 12 hours. The recommended starting dose is 50 mg, and a high-fat meal slows absorption.
- Tadalafil can work from about 30 minutes, peaks at around 2 hours, and its window of effect lasts up to 36 hours. Food has little effect on it, and a low daily dose is an option for some men.
What your doctor checks first
Before prescribing, a doctor confirms that you have ED and looks for related causes such as diabetes, high blood pressure or low testosterone. Cardiac safety is assessed too, because sex is a form of physical exertion. Anyone with chest pain or unstable heart disease needs evaluation first.[1],[2]
Your doctor also needs to know every medicine you take, especially nitrates and nicorandil for angina, which must never be combined with these medicines; alpha-blockers, which need caution; and drugs that change how the liver breaks medicines down. People with kidney or liver disease may need a lower dose.[1]
Comparing them in everyday use
Who each may suit. Sildenafil may suit men who have sex occasionally, can plan ahead and prefer the effect to wear off within the day. Tadalafil may suit men who want flexibility, feel pressured by timing, or are often active over a weekend.[1],[2]
Common side effects include headache, flushing, nasal congestion and indigestion with both. Sildenafil can briefly change colour vision; tadalafil can cause back or muscle aches. Most side effects are mild and ease with continued use.[2]
Getting the best result. Take the dose at a suitable time before sex, make sure there is sexual stimulation, avoid heavy drinking, which itself worsens erections, and do not judge the medicine after a single anxious attempt. If correct use still does not work, come back to adjust the dose or switch medicine. Do not raise the dose or combine the two yourself.[1],[2]
How we care for you at Dr. Kris Clinic
We talk through your real routine, including how often you are sexually active, your work, travel and what matters to you, together with your heart health and current medicines, in English or Thai. Then we recommend a suitable medicine and dose and explain how to use it for the best result.
After you start, we follow up on results and side effects. Any dose change or switch is made step by step. Patients on continuing treatment can request refills through the clinic's system after a doctor's review.
Frequently asked questions
Which one is stronger?
Overall they work about equally well. The main differences are how long they last and their side effects, so the choice depends on your lifestyle and health.
Can I drink alcohol with them?
A small amount is usually fine, but heavy drinking makes erections harder and can add to dizziness from lower blood pressure.
Will I have an erection all the time?
No. They only help when there is sexual stimulation. An erection lasting more than four hours is a medical emergency.
Can I alternate between the two?
Do not take both on the same day. If you want to switch, ask your doctor how to change safely.
Sildenafil and tadalafil share a mechanism and work about equally well, but differ in duration, the effect of food and side effects.[1] Choose with your doctor, never combine medicines yourself, and remember that sexual stimulation is always still needed.
Talk to a doctor about this
Confidential care in Surat Thani, the mainland gateway to Koh Samui, Koh Phangan and Koh Tao. Tue–Fri 17:00–20:00 · Sat–Sun 10:00–15:00 (UTC+7)
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Dr. Krida Uakridathikarn (Dr. Kris)
Board-certified family physician with certificate training in clinical sexology and sexual medicine. He works at Suratthani Hospital and provides sexual health, hormone and LGBTQ+ inclusive care at Dr. Kris Clinic. Profile and CV
References
- EAU Guidelines 2026: Management of Erectile Dysfunction
- EAU Patient Information: Medicines for ED (2026)
- Chung et al. USANZ/AChSHM ED guideline. Med J Aust. 2022;217:318–324. doi:10.5694/mja2.51694
Keep reading
Main guide: Men's sexual health
Erectile Dysfunction (ED): Causes, Tests and Treatments That Work
Erections and Heart Health: Why ED Can Be an Early Warning Sign
What Morning Erections Tell You About Your HealthThis article is general education, not personal medical advice or a promise of results. Diagnosis and treatment require assessment by a doctor. Content reflects the cited sources as of the update date.