Sildenafil 50 mg or 100 mg: Which Dose Suits Whom?
Most men start sildenafil at 50 mg. Learn when a doctor moves to 100 mg or 25 mg, and how to take it so it actually works.
Key points
- Most men start at 50 mg; 100 mg is the daily maximum and 25 mg suits older men or those on interacting drugs.
- Take it about 1 hour before sex, ideally not after a fatty meal, and judge it after 4–6 proper attempts.
- Never combine with nitrates or poppers; go to hospital for an erection over 4 hours or chest pain.
Sildenafil, which many people know as Viagra, has been used to treat erectile dysfunction (ED) since 1998. In Thailand there are several generic brands containing the same active drug. A question I hear often is: "Should I start at 50 or 100?" Let's take a look.
How sildenafil works
Sildenafil is a PDE5 inhibitor. It helps the blood vessels in the penis relax so blood can flow in more easily when you are sexually stimulated. It does not increase desire and does not cause an erection by itself without arousal.
It starts working within 30–60 minutes and has a half-life of about 3–5 hours, so it works best for around 4–5 hours after you take it, shorter than tadalafil. See the comparison in Tadalafil vs sildenafil and other PDE5 inhibitors.
How 50 mg and 100 mg differ
| Topic | 50 mg | 100 mg |
|---|---|---|
| Role | Recommended starting dose for most men | Maximum daily dose |
| Effect on erections | Works well for many men | Can add more effect when 50 mg isn't enough |
| Side effects | Fewer | Headache, flushing, stuffy nose and a blue tinge to vision are more common |
| Suits | First-time use | Moderate to severe ED in men who tolerate it well |
The usual approach is to start at 50 mg, use it properly several times, and then assess. If it still isn't enough and there are no worrying side effects, the doctor may increase it to 100 mg. If side effects are bothersome, it may be reduced to 25 mg.
Your doctor adjusts the dose after seeing real-world results. Not more than once a day.
Who should start at a low dose (25 mg)
- Men aged 65 or over
- Men with liver problems or significantly reduced kidney function
- Men taking CYP3A4 inhibitors, such as some antifungals, erythromycin or HIV medicines containing ritonavir (in the last case both dose and frequency must be limited as the doctor prescribes)
- Men taking alpha-blockers for the prostate or blood pressure
Medicines to be careful with
| Medicine | What you should know |
|---|---|
| Any nitrate, poppers, riociguat | Never combine: blood pressure can drop severely |
| Alpha-blockers such as doxazosin | You should be stable on them first; doctors usually start at 25 mg |
| HIV medicines containing ritonavir | Levels rise sharply; the US label limits sildenafil to 25 mg in 48 hours |
| Other CYP3A4 inhibitors such as ketoconazole, itraconazole, clarithromycin | Doctors usually start at a low dose |
Tell your doctor about every medicine you take, including supplements, because some "sexual enhancement" products are illegally spiked with this group of drugs.
How to take it so it works
- Take it about 1 hour before sexual activity. It can work from 30 minutes up to 4 hours afterwards.
- On an empty stomach or after a light meal. A fatty meal slows absorption so it works later than you intended.
- Avoid large amounts of alcohol, which both makes erections harder and increases the chance of feeling faint.
- You need to be aroused. Taking it and then sitting and waiting won't work; you need stimulation and relaxation.
- Try several times before deciding. The first few times, nerves may stop it working fully. Guidelines suggest judging it after at least 4–6 correct attempts.
- No more than once a day, and not with another PDE5 inhibitor.
If anxiety is a major factor, relaxation practice helps the medicine work better. Read Stress, relaxation and erections.
- 1Take about 1 hour beforeCan be used from 30 minutes to 4 hours before
- 2Empty stomach / light mealVery fatty food slows the effect
- 3Avoid heavy alcoholIt makes erections harder and raises dizziness risk
- 4Be arousedStimulation and relaxation are needed
- 5Try several timesJudge after at least 4-6 correct attempts
Never combine with nitrates or poppers.
How much does a heavy meal delay it?
According to the prescribing information, a high-fat meal delays the time to peak level by about 1 hour and lowers the peak level by about 29%. If you take it after eating, keep the meal light.
If it doesn't work: what to check before increasing the dose
- Have you tried it correctly enough times? Most guidelines suggest at least 4–6 attempts; I usually ask men to try about 6–8 times before deciding.
- Was there real sexual stimulation?
- Timing: did you wait for it to start working, and not leave it beyond the window when it works?
- Did the medicine come from a reliable source?
If all of these are covered and it still isn't enough, your doctor may consider increasing to 100 mg, changing medicine, or looking for other causes such as diabetes or low testosterone.
Possible side effects
- Headache, flushing, stuffy nose
- Indigestion
- Seeing a blue tinge or colours looking different, temporary sensitivity to light, more common at higher doses
- Dizziness
Key contraindications and emergency symptoms
Go to hospital immediately if you have:
- An erection lasting more than 4 hours
- Chest pain or tightness, severe palpitations, fainting or nearly fainting
- Sudden blurred vision or loss of vision, or sudden hearing loss
Read about the dangers of poppers in Poppers: what they are.
A quick comparison with tadalafil
| Topic | Sildenafil | Tadalafil |
|---|---|---|
| Window of effect | About 4–5 hours | Up to 36 hours |
| Fatty food | Delays it | Little effect |
| Typical side effects | Blue-tinged vision, flushing | Back pain, muscle aches |
| Daily option | No | Yes, at a low dose |
Overall effectiveness is similar; the choice depends on your lifestyle and side effects.
Medicine is not the only answer
Sildenafil helps a lot, but if your ED comes from diabetes, high blood pressure, high cholesterol or low testosterone, treating the cause matters just as much. Read What causes erection problems, and if you suspect a hormone problem, see Symptoms of low testosterone.
See a doctor at Dr. Kris Clinic
We review your health history, heart health, current medicines and the cause of your ED before recommending the right dose, then follow up and adjust the plan, at the clinic in Surat Thani or online, with consultations in English available. Don't buy medicine to take on your own from sources you cannot verify, because of the risk of fake products and dangerous combinations with other drugs.
Medicines and services your doctor may consider
- Sidegra sildenafil 50 mg, GPO (4 tabs)฿350Details & order →
- Sidegra sildenafil 100 mg, GPO (4 tabs)฿450Details & order →
- Sinafil sildenafil 50 mg (4 tabs)฿450Details & order →
Prices are per pack. Every medicine is prescribed only after a doctor's assessment; your doctor will advise whether it suits you. See all medicines & services
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Reading is a good start. When you are ready, check where you are, see how a consultation works, then book a time that suits you.
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3Book with the doctorPick a time online or message us on LINE. Come on your own or with your partner. Payment confirms the booking.Book online →Self-checks are a guide only and do not replace a doctor's assessment. Outcomes vary from person to person, depending on the severity of symptoms, the complexity of the problem and other factors.
Worried about sexual performance?
Talk to a doctor in private. We look at physical, hormonal and vascular causes before choosing the treatment that suits you.
Every medicine and service requires a doctor's assessment first.
Frequently asked questions
50 mg didn't work. Should I take another tablet?
No. Never take it more than once a day. First check how you used it, such as food, alcohol and stimulation, then talk to your doctor about adjusting the dose.
How long does sildenafil last?
It works best for about 4–5 hours after you take it. It doesn't keep you hard for that whole time, but helps you get an erection when you are stimulated.
Can older men use it?
Yes, with a doctor's assessment. Older men usually start at a low dose, and heart health, blood pressure and other medicines need to be checked.
I take blood pressure medicine. Can I use sildenafil?
Usually yes, but tell your doctor, because alpha-blockers and some other blood pressure medicines can lower blood pressure further, so your doctor may start you on a low dose. Nitrates are the exception: never combine them.
Which is better, sildenafil or tadalafil?
They work about equally well. Sildenafil suits men who want a short-acting medicine for planned sex, while tadalafil suits those who want more flexibility with timing or who also have prostate symptoms. Your doctor can help you choose.
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References
- US Food and Drug Administration. VIAGRA (sildenafil citrate) tablets prescribing information. Revised 2017
- Goldstein I, et al. Oral sildenafil in the treatment of erectile dysfunction. N Engl J Med. 1998;338(20):1397-1404
- Burnett AL, et al. Erectile Dysfunction: AUA Guideline. J Urol. 2018;200(3):633-641
- European Association of Urology. EAU Guidelines on Sexual and Reproductive Health. 2025
- Chen L, Staubli SEL, Schneider MP, et al. Phosphodiesterase 5 inhibitors for the treatment of erectile dysfunction: a trade-off network meta-analysis. Eur Urol. 2015;68(4):674-680
- US Food and Drug Administration. Tainted sexual enhancement products: consumer warnings. Accessed 2026
This article is for education and does not replace diagnosis or treatment by a doctor. If you have symptoms or concerns, please see a doctor.
