Sildenafil 100mg: When Is It Prescribed and Is It Safe?

Sildenafil 100mg is the highest dose licensed in the UK for erectile dysfunction (ED). It is a prescription-only medicine, so a prescriber decides whether you should move up to it from 50mg. You take one tablet about 1 hour before sex, no more than once in 24 hours.

For most men it is safe when prescribed properly. It is not a self-upgrade, and it is not right for everyone.

How prescribers choose between 25mg, 50mg and 100mg#

The UK product information (the SmPC) sets the usual starting dose at 50mg. Your prescriber can then lower it to 25mg or raise it to 100mg. The choice depends on how well it works and how well you tolerate it.

Some people start on 25mg instead. That includes men over 65, men with severe kidney problems (creatinine clearance under 30 mL/min) and men with liver impairment. It also includes men taking certain medicines that raise sildenafil levels.

A move to 100mg usually follows a few tries at 50mg with only a partial response and few side effects. It is a decision based on your results, not on guesswork.

How much more does 100mg help?#

The landmark trial (Goldstein and colleagues, New England Journal of Medicine, 1998) tested fixed doses in men with ED. It asked how many reported improved erections.

Going from 50mg to 100mg helped about 7 extra men in every 100. Side effects also tend to be more common at higher doses. That is why 50mg is the usual start.

Taking it: timing, food and alcohol#

Swallow the tablet with water about 1 hour before sex. It can work in as little as 30 minutes, and the effect lasts up to 4 to 5 hours. Our guide to how long sildenafil takes to work covers timing in more detail.

A heavy, fatty meal can delay the effect by around an hour. Heavy drinking can make erections harder and add to the dizziness. Large amounts of grapefruit juice may raise sildenafil levels a little, so it is best avoided.

Do not take sildenafil if you#

• take nitrates (such as GTN spray or isosorbide mononitrate), nicorandil, or use poppers (amyl nitrite)
• take riociguat (a pulmonary hypertension medicine)
• are allergic to sildenafil or any ingredient in the tablet
• have low blood pressure (below 90/50 mmHg)
• have had a heart attack or stroke in the last 6 months
• have unstable angina or severe heart failure
• have severe liver disease
• have lost vision in one eye from optic nerve damage (called NAION)
• have an inherited eye condition such as retinitis pigmentosa
• have been told to avoid sex because of your heart

Nitrates and sildenafil together can drop your blood pressure dangerously. Read more in our guide to sildenafil and nitrates.

Medicines that change the dose#

If you take an alpha-blocker (for prostate symptoms or blood pressure), 100mg is not a starting dose. You need to be stable on the alpha-blocker first. Prescribers usually begin sildenafil at 25mg. Tell your prescriber about the alpha-blocker so they can advise you on dizziness when standing up.

Some medicines raise sildenafil levels, including erythromycin, ketoconazole, itraconazole and some HIV medicines. You may need to start at 25mg. With ritonavir, the limit is a single 25mg dose in 48 hours.

Side effects at 100mg#

Headache affects more than 1 in 10 people. These affect up to 1 in 10:

• flushing
• indigestion
• dizziness
• a blocked nose
• vision changes, such as a blue tinge or blurred sight

Most are mild and pass within a few hours. Do not drive or use machinery if you feel dizzy or your vision is affected.

Seek urgent help if you notice any of these:

• an erection lasting more than 4 hours (go to A&E)
• sudden loss of vision in one or both eyes (stop the tablet and get urgent help)
• sudden hearing loss, with or without ringing or dizziness
• chest pain during sex (stop and call 999)

If you have chest pain, do not use GTN spray. Tell the paramedics you have taken sildenafil. You can report side effects through the MHRA Yellow Card scheme.

If you take too much#

Taking a second tablet by mistake, or more than 100mg in a day, makes side effects more likely and stronger. Call NHS 111 or ask a pharmacist for advice. Call 999 or go to A&E if you have chest pain, fainting, vision loss or a prolonged erection.

If 100mg does not seem to work#

Do not double up. Check the basics first:

• Are you aroused? The tablet only helps when you are sexually stimulated.
• Did you leave 30 to 60 minutes after taking it?
• Did you eat a heavy, fatty meal or drink a lot of alcohol?
• Are anxiety or relationship stress getting in the way? NHS Talking Therapies can help.
• Do you take other medicines that affect erections?

Then speak to your GP or prescriber. ED can be an early sign of heart disease, diabetes or low testosterone. A health check may find the cause. They may also suggest tadalafil, which lasts up to 36 hours.

Getting sildenafil 100mg in the UK#

Viagra Connect is a pharmacist-supplied medicine, but only in 50mg. The 100mg strength needs a prescription. On the NHS, a GP can prescribe for ED linked to certain medical conditions listed in NHS regulations, such as diabetes or MS. Otherwise you pay for a private prescription.

Counterfeit sildenafil is common online. Check that any pharmacy is on the GPhC register and shows the GPhC logo. At Totiva, the online consultation is free and you pay only after approval. Sildenafil 100mg costs £7.99 for 4 tablets or £14.99 for 8. You can see the options on our erectile dysfunction page.