Tadalafil and Isoniazid: Can You Take Them Together?

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What is tadalafil and isoniazid used for?

Tadalafil is a prescription medicine used to treat erectile dysfunction (ED) in men. It belongs to a group of medicines called PDE5 inhibitors, which work by relaxing blood vessels in the penis, allowing increased blood flow when you are sexually aroused [3][4]. Tadalafil is available as the branded medicine Cialis or as generic tadalafil. It can be taken on-demand (10 mg or 20 mg at least 30 minutes before sex, with effects lasting up to 36 hours) or as a daily low dose (2.5 mg or 5 mg) [4].

Isoniazid is an antibiotic used to treat and prevent tuberculosis (TB), a serious bacterial infection that usually affects the lungs but can affect other parts of the body [12]. It is often used in combination with other TB medicines such as rifampicin, pyrazinamide, and ethambutol. Treatment typically lasts several months, and it is essential to complete the full course even if you feel better. Isoniazid works by stopping the TB bacteria from growing and multiplying.

Both medicines are processed (metabolised) by the liver, which is why your prescriber will check your liver function and current health status before prescribing tadalafil if you are taking isoniazid.

How do tadalafil and isoniazid interact in the body?

Tadalafil is mainly broken down in the liver by an enzyme system called CYP3A4 [4]. Isoniazid can affect liver enzymes, though it is not classified as a potent CYP3A4 inhibitor or inducer in the way that some other medicines are. This means that isoniazid is unlikely to significantly raise or lower tadalafil levels in your blood under normal circumstances.

However, isoniazid can occasionally cause liver inflammation (hepatitis) or affect liver function, particularly when used with other TB medicines like rifampicin [12]. If your liver is not working as well as it should, your body may process tadalafil more slowly, which could theoretically increase the risk of side effects such as headache, flushing, indigestion, or dizziness [4].

There is no evidence that tadalafil affects how isoniazid works or increases the risk of TB treatment failure. The key concern is ensuring your liver is healthy enough to handle both medicines safely. Your prescriber will review recent blood tests if available and may request liver function tests if there is any concern. This careful assessment is part of the online consultation process at Cured Pharmacy, where a UK-registered prescriber reviews every request for ED treatment.

What about rifampicin and other TB medicines?

If you are taking rifampicin alongside isoniazid (a common combination for TB treatment), it is important to know that rifampicin is a potent CYP3A4 inducer. This means rifampicin can significantly reduce tadalafil levels in your blood, making the ED treatment less effective [4]. Your prescriber will take this into account and may adjust the dose or timing of tadalafil, or discuss alternative ED treatments. Always declare all your TB medicines during your consultation, not just isoniazid.

When to get urgent help

• Erection lasting more than 4 hours (priapism) – go to A&E immediately
• Sudden loss of vision or hearing – seek urgent medical attention
• Chest pain during or after sexual activity – call 999
• Severe dizziness, fainting, or allergic reaction (swelling, difficulty breathing) – call 999
• Yellowing of skin or eyes, dark urine, or severe tiredness – contact your doctor same day

Call 999 or go to A&E in an emergency, or contact NHS 111 for urgent advice.

What does your prescriber check before approving tadalafil and isoniazid together?

When you request tadalafil through Cured Pharmacy's online consultation, you will be asked to list all your current medicines, including isoniazid and any other TB treatments. The prescriber will check several important factors to ensure tadalafil is safe and appropriate for you [2][4].

First, they will review your cardiovascular health. Tadalafil can cause a small drop in blood pressure, and sexual activity itself places demands on the heart [4]. If you have unstable angina, severe heart failure, or have had a recent heart attack or stroke, tadalafil may not be suitable. Second, they will assess your liver and kidney function, especially important if you are on long-term isoniazid, as both organs help clear tadalafil from your body [4].

The prescriber will also check for any contraindicated medicines. You must never take tadalafil with nitrates (such as GTN spray or isosorbide mononitrate), nicorandil, riociguat, or recreational drugs called 'poppers', as the combination can cause a dangerous fall in blood pressure [4]. If you take alpha-blockers for prostate symptoms (such as tamsulosin or doxazosin), the prescriber will ensure you are stable on that medicine and start you on the lowest dose of tadalafil to minimise the risk of dizziness [4].

Finally, the prescriber will consider any other medicines that strongly affect liver enzymes. If you are taking potent CYP3A4 inhibitors (such as ritonavir, ketoconazole, itraconazole, or clarithromycin), the maximum dose of tadalafil will be reduced [4]. This thorough review ensures that tadalafil and isoniazid, along with any other medicines you take, can be used together as safely as possible.

Practical advice for taking tadalafil while on isoniazid

If your prescriber approves tadalafil, follow the dosing instructions carefully. For on-demand use, take tadalafil at least 30 minutes before you plan to have sex; the effect can last up to 36 hours, so you do not need to rush [4]. You still need to be sexually aroused for the medicine to work—it will not cause an automatic erection. Do not take more than one dose in 24 hours.

Continue taking your isoniazid and other TB medicines exactly as prescribed by your TB specialist or GP. Do not stop or change the dose of any TB medicine without medical advice, as incomplete treatment can lead to drug-resistant TB. If you experience new symptoms such as yellowing of the skin or eyes (jaundice), dark urine, persistent nausea, or unusual tiredness, contact your doctor promptly, as these may be signs of liver problems [12].

Avoid drinking large amounts of alcohol while taking tadalafil, as alcohol can worsen erectile dysfunction and add to the blood-pressure-lowering effect, increasing the risk of dizziness or fainting [4]. Grapefruit juice can raise tadalafil levels and should be avoided. If you develop a headache, indigestion, or flushing after taking tadalafil, these are common side effects and usually mild; however, if they are severe or persistent, speak to a pharmacist or doctor.

Store both medicines in a cool, dry place away from children. Keep tadalafil in its original packaging until you are ready to use it. If you have any questions about your medicines or notice anything unusual, contact Cured Pharmacy or your GP for advice.

When to seek urgent help

Seek immediate medical attention if you experience an erection lasting more than four hours (priapism), sudden loss of vision or hearing, or chest pain during sexual activity [4]. These are rare but serious side effects. If you develop severe dizziness, fainting, or signs of an allergic reaction (such as swelling of the face, lips, or throat, or difficulty breathing), call 999 or go to A&E straight away.

At a glance

Can tadalafil and isoniazid affect erectile function or TB treatment?

Isoniazid itself is not known to cause erectile dysfunction directly. However, TB is a serious illness, and the physical and emotional stress of managing a long-term infection can contribute to ED [1][10]. Fatigue, anxiety, and the impact of chronic illness on overall wellbeing may all play a role. Treating the underlying TB and maintaining good general health are important steps in managing ED.

Tadalafil does not interfere with the antibacterial action of isoniazid or reduce the effectiveness of TB treatment. There is no evidence that PDE5 inhibitors like tadalafil affect the immune system or the body's ability to fight infection. You can safely use tadalafil for ED while completing your TB treatment, provided your prescriber has reviewed your full medical history and current medicines.

If you notice that tadalafil is not working as well as expected, consider whether you are also taking rifampicin, as this medicine can reduce tadalafil levels significantly [4]. Speak to your prescriber, who may adjust the dose or suggest an alternative ED treatment. Remember that ED can have multiple causes—including diabetes, high blood pressure, high cholesterol, smoking, and psychological factors—so it is worth seeing your GP for a full assessment if you have not already done so [1][2].

Getting tadalafil safely with Cured Pharmacy

Cured Pharmacy is a GPhC-registered online pharmacy (GPhC 9012511) offering a convenient, confidential service for erectile dysfunction treatment. All requests for prescription ED medicines, including tadalafil, are reviewed by a UK-registered prescriber as part of an online consultation. You will be asked about your medical history, current medicines (including isoniazid and any other TB treatments), and any other health conditions.

If tadalafil is suitable for you, you can choose from a range of options, including generic tadalafil from £7.95 or branded Cialis from £12.00 (first order with code Ednew: £6.36 and £9.60 respectively). Tadalafil 20 mg (Teva) is available at £9.99 (first order: £7.99), and Avarante (tadalafil) 10 mg at £11.49 (first order: £9.19). Your medicine is dispensed by our UK pharmacists and delivered discreetly to your door.

If you prefer a different PDE5 inhibitor, we also offer sildenafil (generic Viagra) from £0.45, vardenafil (generic Levitra) at £16.95, and Spedra (avanafil) at £20.95, all with first-order discounts using code Ednew. Each medicine works in a similar way but has different onset times and durations of action, so your prescriber can help you choose the best option for your lifestyle and health needs [3][4][5][6].

If you have any questions about taking tadalafil and isoniazid together, our pharmacy team is here to help. You can contact us through the website or speak to a pharmacist for advice. We are committed to providing safe, effective, and compassionate care for all our patients.

Why see a GP for new erectile dysfunction?

If you are experiencing erectile dysfunction for the first time, it is important to see your GP for a full assessment. ED can be an early warning sign of cardiovascular disease, diabetes, or other health conditions that need treatment [1][2]. Your GP can arrange blood tests, check your blood pressure and cholesterol, and discuss lifestyle changes such as stopping smoking, losing weight, reducing alcohol, and increasing physical activity—all of which can improve erectile function and overall health [2]. Online ED services like Cured Pharmacy complement, but do not replace, this important initial assessment.

In summary

• No major direct interaction between tadalafil and isoniazid is commonly reported in clinical practice.
• Your prescriber checks liver function and all TB medicines during the online consultation.
• Rifampicin can reduce tadalafil effectiveness; your prescriber may adjust the dose or suggest alternatives.
• Continue your TB treatment as prescribed; tadalafil does not interfere with isoniazid's antibacterial action.
• Seek urgent help for erection lasting over 4 hours, sudden vision or hearing loss, or chest pain during sex.

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