All four common ED tablets (sildenafil, tadalafil, vardenafil and avanafil) work in the same way, so they share many of the same side effects. Most are mild and short-lived. But each has its own pattern, and knowing it can help you and your prescriber choose the one that suits you best.
Why ED tablets cause side effects
These medicines are PDE5 inhibitors. They relax blood vessels so more blood can flow into the penis during arousal. But they widen blood vessels elsewhere in the body too1. That explains the most common side effects: headache, a flushed face and a blocked nose. They can also cause a small, temporary fall in blood pressure1.
The common side effects, medicine by medicine
In the clinical trials, headache was the most common side effect for all four medicines1234. After that, each has its own pattern:
| Medicine | Most common side effects in trials | Worth knowing |
|---|---|---|
| Sildenafil (Viagra) | Headache, flushing, indigestion, changes in vision, blocked nose1 | Vision changes are usually a mild colour tinge, sensitivity to light or blurring, and they pass. They were more common at 100 mg1. |
| Tadalafil (Cialis) | Headache, indigestion, back pain, muscle aches, blocked nose, flushing, pain in the arms or legs2 | Back pain and muscle aches usually start 12 to 24 hours after a dose and settle within 48 hours2. |
| Vardenafil (Levitra) | Headache, flushing, runny or blocked nose, indigestion, sinusitis3 | Can affect the heart’s rhythm on an ECG (the QT interval), so it needs care with some heart rhythm medicines3. |
| Avanafil (Stendra, Spedra) | Headache, flushing, blocked nose, cold symptoms, back pain4 | Taken closer to sex than the others: about 15 minutes before at the usual doses4. |
Side effects tend to be more common at higher doses1. In the trials, only a small number of men stopped treatment because of side effects12.
The NHS also lists dizziness, an upset stomach and, for sildenafil, anxiety as common side effects56. If you feel dizzy or your vision is blurred, do not drive or use machinery until it passes5.
How to reduce the common side effects
- Ask about a lower dose. The labels allow the dose to be lowered if side effects are a problem12, and a lower dose may still work well for you.
- Go easy on alcohol. Alcohol also widens blood vessels. Heavy drinking with tadalafil made dizziness, headache and a drop in blood pressure on standing more likely2.
- Consider switching. If one side effect bothers you, another medicine may suit you better. For example, back ache is more typical of tadalafil, and colour tinge to vision of sildenafil12.
- Talk to a pharmacist if a side effect bothers you or does not go away. They may be able to suggest ways to reduce it5.
Which ED pill has the fewest side effects?
The honest answer is that nobody can say for certain. The labels warn that side-effect rates from one medicine’s trials cannot be directly compared with another’s, because the trials were run in different ways12. European guidelines describe all four as having a good overall safety profile, with no proven difference in how well they work7.
In practice, the right choice depends on which side effect bothers you most, how long you want the effect to last, and your other medicines. For head-to-head comparisons, see sildenafil vs tadalafil, avanafil vs sildenafil and our vardenafil comparison.
Rare but serious: get help straight away
- An erection lasting more than 4 hours. Get emergency help. Left untreated, it can permanently damage the penis1.
- Sudden loss of vision in one or both eyes. Stop taking the medicine and get medical help1. This can be a sign of a rare problem with blood flow to the optic nerve, called NAION.
- Sudden loss or reduction of hearing, sometimes with ringing in the ears or dizziness. Stop and get help promptly1.
- Chest pain, a fast or irregular heartbeat, fainting, or signs of a serious allergic reaction, such as a swollen throat or difficulty breathing. Call emergency services5.
If you get chest pain after taking an ED tablet, tell the emergency team what you took and when. Nitrate medicines for chest pain must not be combined with ED tablets1.
Can ED pills cause blindness?
Sudden vision loss from NAION has been reported rarely in men taking any of these medicines1. NAION is uncommon in any case: the Viagra label puts it at about 2.5 to 11.8 cases per 100,000 men aged 50 and over each year. Studies suggest taking a PDE5 inhibitor may roughly double that small risk for a short time1. If you have ever had NAION, sildenafil may not be suitable for you5.
Side effects that come from mixing medicines
- Nitrates and riociguat: never. The combination can make blood pressure fall dangerously1.
- Blood pressure medicines: usually fine, but they can lower blood pressure a little more. Alpha-blockers, used for blood pressure or the prostate, need extra care and a lower starting dose1.
- Some HIV medicines, antifungals and antibiotics raise levels of ED medicines, which makes side effects more likely1.
Our guide to ED pills, blood pressure and heart disease covers this in more detail.
Long-term use
These medicines have been studied over long periods: more than 550 men took sildenafil for over a year in its trials, and some men took daily tadalafil for two years12. If you take one regularly, keep up with check-ups, and review it with your prescriber if your health or other medicines change.
You can see the options we supply on our ED medicines page, such as Viagra 100 mg and the avanafil tablet Avana 100 mg. If your tablet is not working as hoped, see Viagra not working?
Frequently asked questions
What are the most common side effects of Viagra?
Headache, flushing, indigestion, a blocked nose and temporary changes in vision, such as a colour tinge or sensitivity to light. They are usually mild and pass as the medicine wears off.
What are the side effects of Cialis?
Headache, indigestion, back pain, muscle aches, a blocked nose and flushing. Back and muscle aches usually start 12 to 24 hours after a dose and settle within 48 hours.
Which ED pill has the fewest side effects?
There is no reliable way to rank them, because their trials were run differently. All four have a similar safety profile. The best choice depends on which side effects you notice and how the medicine fits your life.
How do I avoid a headache from sildenafil?
Ask your prescriber about a lower dose, and avoid heavy drinking. If headaches continue, another ED medicine may suit you better.
Can ED pills cause blindness?
Rarely, sudden vision loss has been reported. If you lose vision suddenly in one or both eyes, stop the medicine and get medical help straight away.
What if an erection lasts too long?
An erection lasting more than 4 hours is an emergency. Go to an emergency department, because delay can cause permanent damage.
Sources & references
- US National Library of Medicine, DailyMed. Viagra (sildenafil) prescribing information.
- US National Library of Medicine, DailyMed. Cialis (tadalafil) prescribing information.
- US National Library of Medicine, DailyMed. Vardenafil tablets prescribing information.
- US National Library of Medicine, DailyMed. Stendra (avanafil) prescribing information.
- NHS. Sildenafil (Viagra). Page last reviewed 29 July 2026.
- NHS. Tadalafil. Page last reviewed 13 May 2026.
- European Association of Urology. EAU Guidelines on Sexual and Reproductive Health: Management of erectile dysfunction.
Related reading
- Can you take Viagra every day?
- Can women take Viagra?
- ED medicines overview
- Sildenafil medicines
- Tadalafil medicines
- Vardenafil medicines
- Avanafil medicines
This content is for informational purposes only and is not medical advice. Report side effects to your doctor or pharmacist, and never take ED medicines with nitrates. Used as prescribed by a licensed medical professional.
Jacob T. is a licensed clinical pharmacist with 18 years of experience. He holds a B.Pharm and M.Pharm from Dr. M.G.R. Medical University. At OnlineMeds, he reviews medicine and health content to help ensure it is accurate, evidence-based, and easy to understand.