Sildenafil vs tadalafil (Viagra vs Cialis): how they compare

Sildenafil and tadalafil are both mainly used to treat erection problems (erectile dysfunction). They are widely known by their brand names, Viagra and Cialis. The NHS says both can be bought from a pharmacy for erection problems after speaking with a pharmacist, but are mostly available with a prescription.

The NHS points out that erection problems can be a sign of other serious health conditions such as diabetes or heart problems, and advises a check-up with a doctor within 6 months of starting if you buy either without a prescription.

The NHS says people taking sildenafil for erection problems are usually told to take 1 tablet 1 hour before sex, although the time it takes to work can vary. Tadalafil is described differently: some people may be told to take 1 tablet 30 minutes before sex, and otherwise it is usually taken as 1 tablet at the same time every day.

Both pages state that no more than 1 tablet should be taken a day because the effects can last up to 36 hours. Both are swallowed whole with water. With sildenafil, the NHS says it may take longer to get an erection if you take it after a large meal, and with tadalafil there is no similar food note beyond grapefruit juice.

The NHS lists the same cautions for both. They may not be suitable if you have had an allergic reaction or serious side effect before, have recently had a heart attack or stroke, have low blood pressure or uncontrolled high blood pressure, have liver or kidney problems, have leukaemia or myeloma, have sickle cell anaemia, have ever had vision loss caused by reduced blood flow to the optic nerve, or have a curved or bent penis that makes erection painful or difficult.

Both can interact with heart medicines such as nitrates, epilepsy medicines, medicines for pulmonary hypertension or high blood pressure, some antibiotics (rifampicin, erythromycin and clarithromycin), medicines for enlarged prostate or erectile dysfunction, HIV medicines and antifungals. Poppers (amyl nitrate) and illegal drugs such as amphetamines, ecstasy and cocaine can increase the risk of serious side effects. Tell the prescriber or pharmacist about everything you take.

Most side effects are mild and short-lived. The NHS says that if you feel dizzy or have blurred vision after taking sildenafil, you should not drive, ride a bike or use machinery until symptoms have gone. Tadalafil does not usually affect the ability to drive, though some people report dizziness. For sildenafil, call 999 for signs of anaphylaxis, heart attack or seizure. For tadalafil, call NHS 111 if you think you are having serious side effects, including sudden hearing or eyesight problems.

An erection that lasts too long needs prompt medical attention. The NHS describes painful erections lasting longer than 4 hours for sildenafil among serious side effects and lists prolonged or painful erection for tadalafil.

Which suits a person depends on how often they want to be ready for sex, other health conditions and medicines, and side effects. A pharmacist or doctor can go through this, and a GP can look for underlying causes, including heart problems and diabetes, which the NHS says erection problems can be a sign of.