Tadalafil and Azathioprine: Can They Be Taken Together?

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At a glance

What Is Tadalafil and How Does It Work?

Tadalafil is a prescription medicine used to treat erectile dysfunction (ED) in men [4]. It belongs to a group of medicines called phosphodiesterase type 5 (PDE5) inhibitors, which work by relaxing the blood vessels in the penis, allowing more blood to flow in when you are sexually aroused [1][4]. Tadalafil is the active ingredient in branded Cialis and is also available as a generic medicine at lower cost.

Tadalafil is available in two forms: on-demand tablets (usually 10mg or 20mg taken at least 30 minutes before sexual activity, with effects lasting up to 36 hours) and daily low-dose tablets (2.5mg or 5mg taken once a day) [4]. Unlike some other ED treatments, tadalafil's long duration of action means you don't need to time it as precisely, offering more spontaneity. However, tadalafil only works when you are sexually stimulated—it does not cause an automatic erection [1][4].

What Is Azathioprine and Why Is It Prescribed?

Azathioprine is an immunosuppressant medicine, which means it reduces the activity of your immune system. It is commonly prescribed to prevent organ rejection after a transplant, or to treat autoimmune conditions such as rheumatoid arthritis, inflammatory bowel disease (Crohn's disease or ulcerative colitis), lupus, and certain skin conditions. By dampening the immune response, azathioprine helps to reduce inflammation and prevent the body from attacking its own tissues.

Because azathioprine affects your immune system, it requires careful monitoring by your doctor, including regular blood tests to check your blood count and liver function. Common side effects can include increased susceptibility to infections, nausea, and fatigue. If you are taking azathioprine, it is important that any new medicine—including treatments for erectile dysfunction—is reviewed by a healthcare professional who understands your full medical picture.

Can Azathioprine Cause Erectile Dysfunction?

Azathioprine itself is not commonly listed as a direct cause of erectile dysfunction. However, the underlying conditions for which azathioprine is prescribed—such as chronic inflammatory diseases or the stress of managing a long-term illness—can contribute to ED [1][10]. Fatigue, low mood, and the physical effects of chronic disease all play a role in sexual health. If you have noticed changes in your erections since starting azathioprine or since your diagnosis, it is worth discussing this with your GP, as ED can sometimes be an early sign of other health issues such as cardiovascular disease or diabetes [1][2].

Tadalafil and Azathioprine: Is There an Interaction?

There is no well-documented pharmacological interaction between tadalafil and azathioprine [3][4]. They work through completely different mechanisms in the body: tadalafil acts on blood vessels in the penis, while azathioprine suppresses immune cell activity. Neither medicine is known to significantly affect the metabolism or effectiveness of the other, and they are not processed by the same liver enzyme pathways in a way that would cause a clinically significant interaction.

That said, the absence of a direct drug-drug interaction does not mean tadalafil is automatically suitable for everyone taking azathioprine. Your prescriber will need to consider your overall health, the reason you are taking azathioprine, any other medicines you are on, and whether you have any cardiovascular or other conditions that might make PDE5 inhibitors less safe [2][4]. For example, if you also take medicines for high blood pressure or have heart disease, additional caution is needed. This is why a full consultation is essential before starting tadalafil.

What you need to know

• No major direct interaction between tadalafil and azathioprine is commonly reported in clinical practice.
• A prescriber must review your full medical history and all medicines before approving tadalafil.
• Never take tadalafil with nitrates, nicorandil, riociguat, poppers, or another PDE5 inhibitor.
• Declare all medicines, supplements, and health conditions in your consultation for safe treatment.
• Seek urgent help for erections lasting over four hours or sudden vision or hearing loss.

What Your Prescriber Will Check Before Approving Tadalafil

When you request tadalafil through Cured Pharmacy or any other prescribing service, a UK-registered prescriber will review your medical questionnaire carefully. They will check for any contraindications—conditions or medicines that make tadalafil unsafe. Absolute contraindications include taking nitrates (such as GTN spray or isosorbide mononitrate), nicorandil, riociguat, or recreational drugs known as 'poppers' (amyl or alkyl nitrites), because combining these with tadalafil can cause a dangerous drop in blood pressure [3][4]. You must never take tadalafil with another PDE5 inhibitor at the same time.

Your prescriber will also ask about your heart health, blood pressure, liver and kidney function, any history of stroke or heart attack, and whether you have conditions such as severe heart failure or a history of sudden vision loss (NAION) [2][4]. If you take alpha-blockers (such as tamsulosin or doxazosin) for prostate symptoms, you may need to start on a lower dose of tadalafil to avoid dizziness from low blood pressure. If you are on medicines that strongly inhibit the liver enzyme CYP3A4 (such as ritonavir, ketoconazole, or clarithromycin), your tadalafil dose may need to be adjusted [4]. The prescriber will review all your current medicines, including azathioprine, to ensure tadalafil is safe and appropriate for you.

Declaring All Your Medicines

It is vital that you list every medicine you take—prescription, over-the-counter, herbal remedies, and supplements—in your consultation. Even if you believe a medicine is unrelated to ED, interactions can be unexpected. For instance, St John's wort (a herbal remedy for low mood) can reduce the effectiveness of tadalafil, and grapefruit juice can increase tadalafil levels in your blood [4]. Your prescriber relies on the information you provide to make a safe decision, so complete honesty is essential.

Practical Advice: Taking Tadalafil While on Azathioprine

If your prescriber approves tadalafil, you can take it as directed while continuing your azathioprine as normal. There is no need to separate the doses or take them at different times of day, as they do not interfere with each other. Follow the dosing instructions for tadalafil carefully: on-demand tablets (10mg or 20mg) should be taken at least 30 minutes before sexual activity, and you should not take more than one dose in 24 hours [4]. Daily tadalafil (2.5mg or 5mg) is taken once a day at the same time, regardless of when you plan to have sex.

Because azathioprine can make you more prone to infections and may cause fatigue, it is important to look after your general health. Eating a balanced diet, staying active (as your condition allows), limiting alcohol, and not smoking can all improve your response to ED treatment and support your overall wellbeing [1][2]. Heavy alcohol can worsen erectile dysfunction and add to the blood-pressure-lowering effect of tadalafil, so moderation is advised. If you experience any new or unusual symptoms after starting tadalafil—such as dizziness, chest pain during sex, or any side effects that concern you—contact your prescriber or GP promptly.

When to get urgent help

• Erection lasting longer than four hours (priapism)—go to A&E immediately to prevent permanent damage.
• Sudden loss of vision in one or both eyes—stop tadalafil and seek urgent medical help.
• Sudden decrease or loss of hearing, sometimes with ringing in the ears or dizziness—stop tadalafil and contact a doctor urgently.
• Chest pain during or after sex—do not take more tadalafil; call 999 if pain is severe or does not go away.
• Severe dizziness, fainting, or signs of a serious allergic reaction (swelling of face, lips, or throat, difficulty breathing)—call 999.

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

When to Seek Urgent Help and Ongoing Monitoring

While tadalafil is generally well tolerated, there are some rare but serious side effects that require immediate medical attention. If you develop an erection that lasts longer than four hours (priapism), you must go to A&E straight away, as this can cause permanent damage if not treated promptly [3][4]. Sudden loss of vision in one or both eyes, or sudden decrease or loss of hearing (sometimes with ringing in the ears or dizziness), are also medical emergencies—stop taking tadalafil and seek urgent help [4]. If you experience chest pain during or after sex, do not use more tadalafil and contact your doctor or call 999 if the pain is severe or does not go away.

Continue to attend all your regular monitoring appointments for azathioprine, including blood tests to check your blood count and liver function. If your doctor changes your azathioprine dose, adds a new medicine, or if your health changes (for example, if you develop heart problems or your blood pressure becomes unstable), let your ED prescriber know, as this may affect whether tadalafil remains suitable for you [2]. Open communication between you, your GP, and your prescriber ensures the safest and most effective treatment.

Other ED Treatment Options

If tadalafil is not suitable or does not work well for you, there are other licensed PDE5 inhibitors available, including sildenafil (Viagra), vardenafil (Levitra), and avanafil (Spedra) [3][5][6]. Each has a slightly different onset and duration of action, and one may suit you better than another. Your prescriber can discuss these alternatives. Non-tablet options include Vitaros (alprostadil cream applied to the tip of the penis) and Caverject (alprostadil injection), both of which are prescription-only [7]. Eroxon is a topical gel medical device available without prescription. Your GP can also refer you for specialist assessment if needed, and may explore underlying causes of ED such as hormonal imbalance, diabetes, or cardiovascular disease [1][2].

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