Vardenafil and Isoniazid: Can They Be Taken Together?

Vardenafil and Isoniazid: Understanding the Interaction

Published: 31 October 2026

If you're taking isoniazid for tuberculosis and considering vardenafil for erectile dysfunction, you may wonder whether vardenafil and isoniazid can be taken together safely. The good news is that no major direct interaction between these two medicines is commonly reported in clinical practice. However, every medication combination requires careful assessment by a prescriber, who will review your full medical history and current medicines during an online consultation before prescribing vardenafil.

What Is Vardenafil and How Does It Work?

Vardenafil is a prescription medicine used to treat erectile dysfunction (ED) in adult men. It belongs to a group of medicines called PDE5 inhibitors, which also includes sildenafil (Viagra) and tadalafil (Cialis). Vardenafil works by relaxing blood vessels in the penis, allowing increased blood flow when you are sexually aroused [3][5].

Vardenafil is available in 5mg, 10mg, and 20mg tablets. The usual starting dose is 10mg, taken about 25 to 60 minutes before sexual activity. It can be taken with or without food, though a heavy meal may delay how quickly it works. You should not take vardenafil more than once in 24 hours, and it only works when you are sexually stimulated [5].

Like all PDE5 inhibitors, vardenafil is not suitable for everyone. It must not be taken with nitrates, nicorandil, riociguat, or recreational drugs called 'poppers', as this combination can cause a dangerous drop in blood pressure. Your prescriber will check your suitability during the consultation [5].

What Is Isoniazid and Why Is It Prescribed?

Isoniazid is an antibiotic medicine used primarily to treat and prevent tuberculosis (TB), a serious bacterial infection that usually affects the lungs. It is often prescribed as part of a combination treatment regimen alongside other TB medicines such as rifampicin, pyrazinamide, and ethambutol. Treatment typically lasts several months to ensure the infection is fully cleared.

Isoniazid works by stopping the growth of the bacteria that cause tuberculosis. It is usually taken once daily, either on an empty stomach or with food if it causes stomach upset. Because TB treatment is lengthy and involves multiple medicines, it's important to take isoniazid exactly as prescribed and to complete the full course, even if you start to feel better.

Like all medicines, isoniazid can cause side effects and may interact with other drugs. Common side effects include nausea, tiredness, and tingling or numbness in the hands and feet (peripheral neuropathy). Your doctor will monitor your liver function during treatment, as isoniazid can occasionally affect the liver. Always tell your prescriber about all medicines you are taking, including over-the-counter products and supplements.

Can Vardenafil and Isoniazid Be Taken Together?

There is no well-documented major interaction between vardenafil and isoniazid in the medical literature. These two medicines work through different mechanisms in the body and are not known to directly interfere with each other's effectiveness or safety. However, this does not mean the combination is automatically suitable for everyone.

During your online consultation, the prescriber will review your full medication list, including isoniazid and any other TB medicines you are taking. Some TB treatments, particularly rifampicin, can affect how the body processes vardenafil, potentially making it less effective. Rifampicin is a potent enzyme inducer that speeds up the breakdown of many medicines, including PDE5 inhibitors. If you are taking rifampicin alongside isoniazid, your prescriber will take this into account [5].

It's also important to consider your overall health. Tuberculosis is a serious infection that can affect your general wellbeing, energy levels, and cardiovascular health. Your prescriber will assess whether you are well enough for sexual activity and whether vardenafil is appropriate for you at this time. Never start, stop, or change any prescribed medicine without consulting your doctor or pharmacist first.

What About Other TB Medicines?

If you are taking rifampicin as part of your TB treatment, this is more likely to affect vardenafil than isoniazid alone. Rifampicin is a CYP3A4 enzyme inducer, which means it speeds up the breakdown of vardenafil in the liver, potentially reducing its effectiveness. Your prescriber may need to adjust your dose or consider an alternative ED treatment if you are on rifampicin [5].

Other TB medicines such as pyrazinamide and ethambutol are not known to have significant interactions with vardenafil. However, it is essential to declare every medicine you are taking during your consultation so that the prescriber can make a fully informed decision about your suitability for vardenafil.

What Does Your Prescriber Check Before Prescribing Vardenafil?

Before prescribing vardenafil, a UK prescriber will carry out a thorough assessment of your medical history and current health. This includes asking about any heart conditions, blood pressure problems, liver or kidney disease, and any medicines you are currently taking, including isoniazid and other TB treatments. This assessment ensures that vardenafil is safe and appropriate for you [2][5].

The prescriber will specifically check for any contraindications, such as the use of nitrates, nicorandil, riociguat, or another PDE5 inhibitor. They will also assess whether you are taking medicines that could interact with vardenafil, such as alpha-blockers (used for prostate problems or high blood pressure) or potent CYP3A4 inhibitors like certain antifungals or HIV medicines. If you are on rifampicin, this will be flagged as a potential concern [5].

You will also be asked about your cardiovascular health and whether you are fit enough for sexual activity. Conditions such as recent heart attack, stroke, unstable angina, or severe heart failure may mean that vardenafil is not suitable. The prescriber may recommend that you see your GP for a cardiovascular assessment before starting treatment, particularly if you have risk factors such as diabetes, high cholesterol, or a history of smoking [2].

Why Declare All Your Medicines?

It is essential to provide a complete and accurate list of all medicines you are taking, including prescription drugs, over-the-counter products, herbal remedies, and supplements. Some interactions are not immediately obvious, and even seemingly unrelated medicines can affect how vardenafil works or increase the risk of side effects.

For example, if you are taking medicines for high blood pressure alongside vardenafil, there is a risk of an additive blood-pressure-lowering effect, which can cause dizziness or fainting. Your prescriber needs to know about these medicines to ensure your safety and to advise you on how to take vardenafil appropriately.

Practical Advice for Taking Vardenafil with Isoniazid

If your prescriber approves vardenafil while you are taking isoniazid, follow their instructions carefully. Take vardenafil about 25 to 60 minutes before you plan to have sex, and do not take more than one dose in 24 hours. Vardenafil will only work if you are sexually aroused, so it is not an automatic trigger for an erection [5].

Avoid drinking large amounts of alcohol when taking vardenafil, as alcohol can make erectile dysfunction worse and increase the risk of side effects such as dizziness, headache, and low blood pressure. Grapefruit juice should also be avoided, as it can increase the levels of vardenafil in your blood and raise the risk of side effects [5].

If you experience any side effects while taking vardenafil, such as severe headache, dizziness, visual disturbances, or chest pain, stop taking the medicine and seek medical advice promptly. If you develop an erection that lasts longer than four hours (priapism), go to A&E immediately, as this is a medical emergency that requires urgent treatment to prevent permanent damage [5].

Continue taking your isoniazid and other TB medicines exactly as prescribed by your doctor. Do not stop or change your TB treatment without medical advice, even if you experience side effects. Completing the full course of TB treatment is essential to cure the infection and prevent the development of drug-resistant bacteria.

Alternatives to Vardenafil for Erectile Dysfunction

If vardenafil is not suitable for you, or if you experience side effects, there are other treatment options for erectile dysfunction. Sildenafil (generic Viagra) and tadalafil (generic Cialis) are alternative PDE5 inhibitors that work in a similar way to vardenafil. Sildenafil is taken about one hour before sex and lasts for around four to six hours, while tadalafil can be taken on-demand (working for up to 36 hours) or as a lower daily dose [3][4].

Avanafil (Spedra) is another PDE5 inhibitor that works more quickly than the others, often within 15 to 30 minutes. Like vardenafil, all PDE5 inhibitors require a prescription and a clinical assessment to ensure they are safe for you. Your prescriber will discuss which option is most suitable based on your health, lifestyle, and preferences [6].

Non-tablet treatments are also available. Eroxon is a topical gel that can be bought without a prescription and works by creating a cooling then warming sensation to stimulate blood flow. Vitaros (alprostadil cream) and Caverject (alprostadil injection) are prescription-only treatments that work locally rather than affecting the whole body, and may be an option if tablets are not suitable [7].

If erectile dysfunction is new or worsening, it is important to see your GP for a full assessment. ED can be an early warning sign of underlying health conditions such as heart disease, diabetes, or high blood pressure, all of which benefit from early detection and treatment [1][2].

How Cured Pharmacy Can Help

Cured Pharmacy is a GPhC-registered online pharmacy (GPhC 9012511) offering discreet, convenient access to erectile dysfunction treatments. All prescription medicines are supplied only after an online consultation reviewed by a UK-registered prescriber, ensuring your safety and suitability for treatment.

We stock a range of ED treatments, including vardenafil, sildenafil, tadalafil, and avanafil, at transparent prices. Your medication is delivered discreetly to your door, and our team of pharmacists is available to answer any questions you may have about your treatment.

Scientific References

1. NHS. (n.d.). Erectile dysfunction (impotence). NHS website. https://www.nhs.uk/conditions/erection-problems-erectile-dysfunction/ [accessed 31 October 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 31 October 2026]
3. Joint Formulary Committee. (n.d.). Sildenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/sildenafil/ [accessed 31 October 2026]
4. Joint Formulary Committee. (n.d.). Tadalafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/tadalafil/ [accessed 31 October 2026]
5. Joint Formulary Committee. (n.d.). Vardenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/vardenafil/ [accessed 31 October 2026]
6. Joint Formulary Committee. (n.d.). Avanafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/avanafil/ [accessed 31 October 2026]
7. Joint Formulary Committee. (n.d.). Alprostadil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/alprostadil/ [accessed 31 October 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.