Tadalafil and Lopinavir: Can They Be Taken Together?

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Tadalafil and Lopinavir: Understanding the Interaction

Published: 22 October 2026

Tadalafil and lopinavir can interact because lopinavir significantly increases the levels of tadalafil in your body, which may raise the risk of side effects. Lopinavir is an HIV medication (protease inhibitor) often combined with ritonavir in treatments such as Kaletra, and it strongly inhibits the liver enzyme (CYP3A4) that breaks down tadalafil. This interaction does not mean the combination is forbidden, but your prescriber will need to adjust your tadalafil dose and monitor you carefully to ensure safe and effective treatment for erectile dysfunction.

What is tadalafil and lopinavir?

Tadalafil is a prescription medicine used to treat erectile dysfunction (ED) in men. It belongs to a group of medicines called phosphodiesterase type 5 (PDE5) inhibitors, which work by relaxing blood vessels in the penis to improve blood flow during sexual stimulation [3][4]. Tadalafil is available as generic tablets and under the brand name Cialis. 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 lower daily dose (2.5 mg or 5 mg) for continuous treatment [4].

Lopinavir is an antiretroviral medicine used to treat HIV infection. It is a protease inhibitor that stops the virus from multiplying in your body. Lopinavir is always given together with ritonavir (another protease inhibitor) in a combination product called Kaletra, because ritonavir boosts lopinavir's effectiveness by slowing down how quickly it is broken down in the liver. This combination is part of highly active antiretroviral therapy (HAART) and is essential for controlling HIV and maintaining your immune system.

Both medicines are metabolised (broken down) by the same liver enzyme system, CYP3A4, which is why they can interact when taken together. Understanding this interaction is important for anyone living with HIV who also experiences erectile dysfunction, as ED is more common in men with chronic health conditions [1][2].

How do tadalafil and lopinavir interact?

Lopinavir (boosted with ritonavir) is a potent inhibitor of the CYP3A4 enzyme in your liver. This enzyme is responsible for breaking down tadalafil and clearing it from your body. When you take lopinavir and tadalafil together, lopinavir slows down the breakdown of tadalafil, causing tadalafil levels in your blood to rise significantly and stay elevated for longer [4][12].

In plain terms, your body cannot clear tadalafil as quickly as usual, so the medicine becomes stronger and lasts longer. This increases the chance of side effects such as headache, flushing, indigestion, back pain, muscle aches, and dizziness. More seriously, higher tadalafil levels can lead to a greater drop in blood pressure, which may cause fainting or dizziness, especially if you are also taking other medicines that lower blood pressure [4].

The interaction is predictable and well-documented, so it can be managed safely by adjusting the dose of tadalafil. Your prescriber will take your HIV treatment into account during your consultation and recommend a lower starting dose or a reduced maximum dose of tadalafil to keep you safe while still providing effective treatment for erectile dysfunction [4][12].

Why ritonavir matters

Ritonavir is included in lopinavir combinations specifically because it is an even more powerful CYP3A4 inhibitor. It boosts lopinavir levels but also amplifies the interaction with tadalafil. Even small doses of ritonavir (used as a 'booster' in many modern HIV regimens) can significantly raise tadalafil levels, so your prescriber must know about any ritonavir-containing medicine you take, including newer combinations such as darunavir/ritonavir or atazanavir/ritonavir [4][12].

What your prescriber will check before prescribing tadalafil and lopinavir together

When you request tadalafil through an online consultation at Cured Pharmacy, you will be asked to list all your current medicines, including your HIV treatment. Your prescriber will review your full medication list to identify any interactions, including lopinavir or other protease inhibitors. They will check the dose and formulation of your HIV medicines, because the strength of the interaction depends on whether ritonavir is included and at what dose [4][12].

Your prescriber will also ask about your blood pressure, heart health, and any history of dizziness or fainting. Because tadalafil can lower blood pressure, and lopinavir may add to this effect, it is important to ensure your cardiovascular system is stable. If you take other blood-pressure medicines (such as alpha-blockers like tamsulosin or doxazosin), your prescriber will assess the combined risk and may start you on the lowest possible dose of tadalafil [3][4].

You will be asked about any other medicines that interact with tadalafil, such as nitrates (GTN spray, isosorbide mononitrate), nicorandil, or riociguat, which must never be taken with PDE5 inhibitors because the combination can cause a dangerous drop in blood pressure. Your prescriber will also check for any history of serious side effects from tadalafil or related medicines, such as sudden vision loss (NAION) or priapism (a prolonged, painful erection) [3][4].

Based on this assessment, your prescriber will decide whether tadalafil is suitable for you and, if so, what dose is safe. For men taking lopinavir/ritonavir, the usual recommendation is to start with a lower dose and not exceed a reduced maximum dose. For on-demand tadalafil, the maximum dose is typically 10 mg in any 72-hour period (rather than the standard 20 mg). For daily tadalafil, the dose may be capped at 2.5 mg once daily [4][12].

Dose adjustments for tadalafil and lopinavir

If you are taking lopinavir (with ritonavir), your prescriber will adjust your tadalafil dose to account for the increased drug levels in your body. For on-demand use, you may be started on tadalafil 10 mg, and you should not take more than 10 mg in any 72-hour period. This is lower than the standard maximum of 20 mg and ensures that tadalafil levels do not build up to unsafe levels [4][12].

If you prefer daily tadalafil for continuous treatment, your prescriber may recommend starting with 2.5 mg once daily. The maximum daily dose is usually capped at 2.5 mg when you are taking a potent CYP3A4 inhibitor like lopinavir/ritonavir. This lower dose still provides benefit for erectile dysfunction while minimising the risk of side effects [4][12].

It is essential that you follow the prescribed dose exactly and do not take extra tablets, even if you feel the effect is not strong enough. Taking more tadalafil than recommended can lead to serious side effects, including a dangerous drop in blood pressure, prolonged erection (priapism), or visual disturbances. If you feel your current dose is not working, speak to your prescriber or pharmacist rather than adjusting the dose yourself [4].

Never take tadalafil more frequently than advised. Because lopinavir slows down how quickly tadalafil leaves your body, the medicine stays active for longer, and taking another dose too soon can cause levels to accumulate. Always wait the full recommended time between doses (at least 72 hours for on-demand tadalafil when taking lopinavir) [4][12].

What if I'm on a different HIV medicine?

Many modern HIV treatments use integrase inhibitors (such as dolutegravir, bictegravir, or raltegravir) rather than protease inhibitors. These medicines do not significantly interact with tadalafil, so standard doses can usually be used. However, some regimens still include a protease inhibitor boosted with ritonavir or cobicistat (another potent CYP3A4 inhibitor), so you must always declare your full HIV treatment during your consultation. Your prescriber will check the specific interaction for your regimen and adjust your tadalafil dose accordingly [4][12].

Side effects and warning signs when taking tadalafil and lopinavir

Common side effects of tadalafil include headache, facial flushing, indigestion, nasal congestion, back pain, and muscle aches. These are usually mild and short-lived, but they may be more noticeable or last longer when you are also taking lopinavir, because tadalafil levels are higher [4]. If side effects are troublesome, speak to your prescriber about adjusting your dose.

Dizziness or light-headedness can occur, especially when you stand up quickly, because tadalafil lowers blood pressure. This effect may be more pronounced when combined with lopinavir or if you take other blood-pressure medicines. Avoid sudden changes in posture, and sit or lie down if you feel dizzy. Do not drive or operate machinery if you feel unsteady [4].

Seek urgent medical help (call 999 or go to A&E) if you experience an erection lasting more than four hours (priapism), sudden loss of vision in one or both eyes, sudden decrease or loss of hearing (sometimes with ringing in the ears or dizziness), or chest pain during or after sex. Chest pain during sex should be treated as a medical emergency; do not take nitrate medicines (such as GTN spray) if you have used tadalafil within the past 48 hours, as this combination can be life-threatening [3][4].

If you develop severe dizziness, fainting, or an unusually fast or slow heartbeat, stop taking tadalafil and seek medical advice immediately. These symptoms may indicate that your blood pressure has dropped too low. Always tell any doctor or paramedic treating you that you have taken tadalafil and are on HIV treatment, so they can avoid giving you medicines that interact dangerously [4].

Practical advice for using tadalafil and lopinavir safely

Take your tadalafil exactly as prescribed, and do not adjust the dose or frequency without consulting your prescriber. Keep a record of when you take each dose, especially if using on-demand tadalafil, to ensure you do not take it more often than recommended (no more than once every 72 hours when taking lopinavir) [4].

Continue taking your HIV medicines exactly as prescribed by your HIV specialist. Do not stop or change your antiretroviral treatment without medical advice, as this can lead to drug resistance and harm your health. If you have any concerns about interactions or side effects, speak to your HIV clinic pharmacist or doctor, who can liaise with the prescriber managing your erectile dysfunction treatment [12].

Avoid drinking large amounts of alcohol when taking tadalafil, as alcohol can make erectile dysfunction worse and increase the risk of dizziness and low blood pressure. Grapefruit juice can also raise tadalafil levels and should be avoided [4]. Tadalafil requires sexual stimulation to work, so it is not an aphrodisiac and will not cause an erection without arousal.

Store your medicines safely and separately, and check the expiry dates. Keep tadalafil in its original packaging, away from moisture and heat. If you experience new or worsening erectile dysfunction, or if your HIV treatment changes, contact your prescriber for a medication review. Erectile dysfunction can sometimes be an early sign of other health problems, such as cardiovascular disease or diabetes, so it is worth discussing with your GP if ED is a new symptom [1][2].

At Cured Pharmacy, all ED treatments are supplied following an online consultation reviewed by a UK-registered prescriber. You can request tadalafil and other PDE5 inhibitors through our secure service, and our clinical team will check your HIV medication and other health information to ensure safe prescribing. If you have questions about your treatment, our pharmacists are available to help.

Can I switch to a different ED medicine?

Other PDE5 inhibitors, such as sildenafil (Viagra), vardenafil (Levitra), and avanafil (Spedra), also interact with lopinavir, though the degree and management differ. Sildenafil requires a lower starting dose (25 mg) and longer intervals between doses when taken with potent CYP3A4 inhibitors [3]. Vardenafil and avanafil are generally not recommended or are contraindicated with strong inhibitors like lopinavir/ritonavir [5][6]. Your prescriber will discuss which option is safest and most suitable for you based on your full medication list and health history.

Scientific References

1. NHS. (n.d.). Erectile dysfunction (impotence). NHS website. https://www.nhs.uk/conditions/erection-problems-erectile-dysfunction/ [accessed 22 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 22 October 2026]
3. Joint Formulary Committee. (n.d.). Sildenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/sildenafil/ [accessed 22 October 2026]
4. Joint Formulary Committee. (n.d.). Tadalafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/tadalafil/ [accessed 22 October 2026]
5. Joint Formulary Committee. (n.d.). Vardenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/vardenafil/ [accessed 22 October 2026]
6. Joint Formulary Committee. (n.d.). Avanafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/avanafil/ [accessed 22 October 2026]
7. Electronic Medicines Compendium. (n.d.). Summaries of Product Characteristics and Patient Information Leaflets. https://www.medicines.org.uk/emc/ [accessed 22 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.