Vardenafil and Phenytoin: Can They Be Taken Together?

Vardenafil and Phenytoin: Understanding the Interaction

Published: 3 November 2026

Vardenafil and phenytoin can be taken together, but phenytoin may reduce how well vardenafil works for erectile dysfunction. Phenytoin is an anticonvulsant medicine used to control epilepsy, and it speeds up the breakdown of vardenafil in your body, potentially making the ED treatment less effective. This interaction is not dangerous, but it's important that your prescriber knows about all your medicines so they can choose the best treatment option and dose for you.

What is vardenafil and how does it work?

Vardenafil (also known by the brand name Levitra) is a prescription medicine used to treat erectile dysfunction in men [5]. It belongs to a group of medicines called PDE5 inhibitors, which work by relaxing blood vessels in the penis, allowing more blood to flow in when you're sexually aroused. This helps you get and keep an erection suitable for sexual activity.

Vardenafil is available in 5mg, 10mg, and 20mg tablets. You take it about 25 to 60 minutes before you plan to have sex, and it works only when you're sexually stimulated [5]. The effects can last for several hours, but you should not take it more than once in 24 hours. Like all PDE5 inhibitors, vardenafil needs to be broken down (metabolised) by enzymes in your liver, particularly one called CYP3A4.

Because vardenafil relies on this liver enzyme, other medicines that affect CYP3A4 can change how well it works. Some medicines slow down the enzyme (making vardenafil levels rise), while others speed it up (making vardenafil break down faster and become less effective). This is where the interaction with phenytoin becomes relevant.

What is phenytoin and why is it prescribed?

Phenytoin is an anticonvulsant (anti-epileptic) medicine used to prevent and control seizures in people with epilepsy. It works by stabilising electrical activity in the brain, reducing the likelihood of abnormal bursts of electrical signals that cause seizures. Phenytoin has been used for many decades and remains an important treatment for various types of epilepsy.

This medicine is usually taken once or twice daily as tablets or capsules, and it's important to take it regularly at the same times each day to keep a steady level in your blood. Your doctor will monitor your phenytoin levels with blood tests, because the amount needed varies from person to person and the difference between an effective dose and too much can be quite small.

Phenytoin is what's known as a CYP3A4 inducer. This means it speeds up the activity of the CYP3A4 enzyme in your liver, causing your body to break down certain other medicines more quickly than usual. This can reduce the effectiveness of those medicines, including vardenafil and other treatments for erectile dysfunction.

How phenytoin affects other medicines

Because phenytoin increases the activity of liver enzymes, it can reduce blood levels of many different medicines, including some contraceptives, blood thinners, and heart medicines. This is why it's essential to tell every doctor and pharmacist that you're taking phenytoin, so they can adjust doses or choose alternative treatments when necessary. The interaction develops gradually over a few weeks as phenytoin builds up in your system and continues for some time after you stop taking it.

How do vardenafil and phenytoin interact?

The interaction between vardenafil and phenytoin happens because phenytoin speeds up the CYP3A4 enzyme that breaks down vardenafil in your liver [5]. When this enzyme works faster, vardenafil is removed from your body more quickly, which means lower levels of the medicine reach your bloodstream and the levels don't stay high for as long. The practical result is that vardenafil may not work as well, or may not work at all, for treating erectile dysfunction.

This type of interaction is called a pharmacokinetic interaction – it's about how the body processes the medicine, rather than a direct clash that causes dangerous side effects. Taking vardenafil and phenytoin together is not considered unsafe in terms of serious harm, but the reduced effectiveness means you might not get the benefit you need from the ED treatment.

The extent of the interaction can vary from person to person, depending on your phenytoin dose, how long you've been taking it, and individual differences in how your liver works. Some men may find vardenafil still helps to some degree, while others may notice little or no effect. This is why your prescriber needs to know about your phenytoin use before recommending vardenafil or any other ED treatment.

What does your prescriber check when you take vardenafil and phenytoin?

When you request vardenafil through an online consultation at Cured Pharmacy, you'll be asked to list all your current medicines, including phenytoin. A UK-registered prescriber reviews every consultation to check for interactions and make sure the treatment is suitable and safe for you. If you're taking phenytoin, the prescriber will consider whether vardenafil is likely to be effective enough, or whether a different ED treatment might work better.

Your prescriber will also review your epilepsy control and overall health. Erectile dysfunction can sometimes be an early warning sign of cardiovascular disease or diabetes [1], so if this is a new problem, you should see your GP for a full assessment. The prescriber will check that you don't have any conditions that make PDE5 inhibitors unsuitable, such as recent heart attack or stroke, severe heart failure, very low blood pressure, or use of nitrate medicines (which must never be combined with any PDE5 inhibitor).

If vardenafil is prescribed despite the interaction with phenytoin, you may be advised to try the highest licensed dose (20mg) to see if it provides benefit, or the prescriber may suggest an alternative ED medicine that is less affected by enzyme inducers. You should never increase the dose yourself or take vardenafil more than once a day, even if it doesn't seem to work well.

Alternative ED treatments if vardenafil doesn't work

If vardenafil proves ineffective because of the phenytoin interaction, your prescriber might suggest trying sildenafil or tadalafil, which are also PDE5 inhibitors but may respond differently [3][4]. However, all PDE5 inhibitors are affected to some extent by CYP3A4 inducers like phenytoin. Non-tablet options such as Vitaros (alprostadil cream) or Caverject (alprostadil injection) work by a completely different mechanism and are not affected by phenytoin, so they may be considered if tablets are not effective [7].

Practical advice for taking vardenafil and phenytoin together

If your prescriber approves vardenafil while you're taking phenytoin, take the vardenafil tablet about 25 to 60 minutes before sexual activity, with or without food (though a heavy, high-fat meal may delay how quickly it works) [5]. Swallow the tablet whole with water. Do not take more than one dose in 24 hours, even if the first dose doesn't seem effective – taking extra will not overcome the interaction and increases the risk of side effects.

Continue taking your phenytoin exactly as your doctor has prescribed. Never stop or reduce phenytoin to try to make vardenafil work better, as this could lead to seizures. If you find vardenafil isn't helping your erections, contact the prescriber or your GP to discuss alternatives rather than simply taking more tablets or giving up on treatment altogether.

Be aware of common side effects of vardenafil, which can include headache, flushing, indigestion, nasal congestion, and dizziness [5]. These are usually mild and short-lived. Avoid drinking large amounts of alcohol when using vardenafil, as alcohol can worsen erectile dysfunction and add to the blood-pressure-lowering effect, increasing dizziness. Grapefruit juice can raise vardenafil levels and should be avoided.

When to seek medical help and warning signs with vardenafil and phenytoin

Seek urgent medical help (call 999 or go to A&E) if you experience an erection that lasts longer than four hours (priapism), even if it's painful. This is a medical emergency that requires immediate treatment to prevent permanent damage. Also seek emergency help if you have sudden loss of vision in one or both eyes, or sudden decrease or loss of hearing, which may be accompanied by ringing in the ears or dizziness – these are rare but serious side effects [5].

If you develop chest pain during or after sexual activity, stop and seek medical help. Do not take nitrate medicines (such as GTN spray) for chest pain if you have used vardenafil in the past 24 hours, as the combination can cause a dangerous drop in blood pressure. Make sure anyone treating you knows you have taken vardenafil.

Contact your GP or prescriber if you notice any increase in seizure frequency after starting vardenafil, though this is not a known effect of the interaction. Also get advice if you experience severe or persistent side effects from vardenafil, or if you're starting any new medicines while taking both phenytoin and vardenafil, as further interactions are possible. Remember that erectile dysfunction can be an early sign of heart disease or diabetes, so if ED is a new problem, a full assessment by your GP is important [1][2].

Medicines you must never take with vardenafil

Do not take vardenafil if you use nitrate medicines for angina (such as GTN spray, isosorbide mononitrate, or isosorbide dinitrate), nicorandil, riociguat, or recreational drugs called 'poppers' (amyl or alkyl nitrites) [5]. The combination can cause a severe and dangerous drop in blood pressure. Do not take vardenafil together with another PDE5 inhibitor such as sildenafil, tadalafil, or avanafil. If you take alpha-blocker medicines for prostate problems or high blood pressure, you must tell your prescriber, as dose adjustments may be needed to reduce the risk of dizziness and fainting.

Scientific References

1. NHS. (n.d.). Erectile dysfunction (impotence). NHS website. https://www.nhs.uk/conditions/erection-problems-erectile-dysfunction/ [accessed 3 November 2026]
2. National Institute for Health and Care Excellence. (n.d.). Erectile dysfunction. Clinical Knowledge Summaries. https://cks.nice.org.uk/topics/erectile-dysfunction/ [accessed 3 November 2026]
3. Joint Formulary Committee. (n.d.). Sildenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/sildenafil/ [accessed 3 November 2026]
4. Joint Formulary Committee. (n.d.). Tadalafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/tadalafil/ [accessed 3 November 2026]
5. Joint Formulary Committee. (n.d.). Vardenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/vardenafil/ [accessed 3 November 2026]
6. Joint Formulary Committee. (n.d.). Alprostadil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/alprostadil/ [accessed 3 November 2026]

Information on this page is for educational purposes only and is not medical advice. Prescription treatments require clinical assessment by a UK-registered prescriber. Always consult a qualified healthcare professional before starting or changing any medication.