Sildenafil and Levothyroxine: Can You Take Them Together?

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Sildenafil and Levothyroxine: Understanding the Interaction

Published: 16 October 2026

Many men taking levothyroxine for an underactive thyroid wonder whether sildenafil and levothyroxine can be taken together safely. The good news is that no major direct interaction is commonly reported between these two medicines, and they are often prescribed together. However, every person's situation is different, which is why a prescriber will review all your current medicines—including levothyroxine—during an online consultation before approving sildenafil for erectile dysfunction.

What is sildenafil and levothyroxine used for?

Sildenafil 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]. Sildenafil is the generic version of Viagra and is available in 25 mg, 50 mg, and 100 mg tablets. You typically take it about one hour before sexual activity, and it can help you get and keep an erection sufficient for sex. Sexual stimulation is still needed for the medicine to work.

Levothyroxine is a thyroid hormone replacement used to treat hypothyroidism (an underactive thyroid gland). When your thyroid does not produce enough thyroid hormone, you may feel tired, gain weight, feel cold, and experience other symptoms. Levothyroxine replaces the missing hormone and is usually taken once a day, ideally on an empty stomach in the morning. It is a long-term treatment that most people take for life to keep their thyroid hormone levels normal.

Both medicines are commonly prescribed in the UK, and many men take them together without problems. However, it is important that your prescriber knows about every medicine you take, including levothyroxine, so they can check for any potential issues and ensure the doses are appropriate for you.

Is there an interaction between sildenafil and levothyroxine?

No major direct interaction is commonly reported between sildenafil and levothyroxine. These two medicines work in different ways and are processed differently by the body, so they do not usually interfere with each other. Sildenafil is broken down mainly by liver enzymes (CYP3A4 and CYP2C9), while levothyroxine is a synthetic version of a natural hormone and does not significantly affect these enzymes [3].

However, it is worth noting that untreated or poorly controlled hypothyroidism itself can contribute to erectile dysfunction. Low thyroid hormone levels can reduce libido, cause fatigue, and affect blood flow, all of which can make it harder to get or maintain an erection. If your thyroid levels are not well controlled, treating the underlying thyroid problem with levothyroxine may improve ED symptoms over time.

During your online consultation, the prescriber will ask about your thyroid condition, your current levothyroxine dose, and whether your thyroid levels are stable. They will also check for other medicines or health conditions that might affect whether sildenafil is safe for you. Even though sildenafil and levothyroxine do not interact directly, the prescriber needs the full picture to make sure treatment is appropriate and safe.

Why hypothyroidism can cause erectile dysfunction

Hypothyroidism can affect sexual function in several ways. Low thyroid hormone levels can reduce testosterone, lower energy and mood, and slow down metabolism, all of which can contribute to erectile difficulties. Some studies suggest that men with untreated hypothyroidism are more likely to experience ED. Once thyroid hormone levels are corrected with levothyroxine, some men find their erectile function improves naturally. However, if ED persists despite stable thyroid levels, sildenafil or another ED treatment may help.

What does the prescriber check before approving sildenafil?

Before prescribing sildenafil, a UK prescriber will review your full medical history and all current medicines, including levothyroxine. They will check whether you have any conditions that make sildenafil unsuitable, such as recent heart attack or stroke, unstable angina, severe heart failure, very low blood pressure, or a history of sudden vision loss (NAION) [1][3].

The prescriber will also look for medicines that must never be taken with sildenafil. These include nitrates (such as GTN spray, isosorbide mononitrate, or isosorbide dinitrate), nicorandil, riociguat, and amyl or alkyl nitrites (poppers). Combining sildenafil with any of these can cause a dangerous drop in blood pressure. If you take another PDE5 inhibitor (such as tadalafil, vardenafil, or avanafil), you must not take sildenafil at the same time.

Other medicines require caution. Alpha-blockers (such as tamsulosin or doxazosin) for prostate symptoms can lower blood pressure, so you should be stable on the alpha-blocker and start sildenafil at the lowest dose. Potent CYP3A4 inhibitors (such as ritonavir, ketoconazole, or clarithromycin) can raise sildenafil levels, so a lower starting dose may be needed. Blood pressure medicines can add to the blood-pressure-lowering effect of sildenafil, causing dizziness, but this is usually manageable. The prescriber will consider all these factors and decide on the safest dose for you.

Practical advice for taking sildenafil with levothyroxine

If you are prescribed sildenafil while taking levothyroxine, there are a few practical tips to help you get the best results. First, continue taking your levothyroxine exactly as your GP has prescribed. Do not stop or change the dose without medical advice, as stable thyroid levels are important for your overall health and may also help with erectile function.

Take sildenafil about one hour before you plan to have sex. It works best on an empty stomach or after a light meal; a heavy or high-fat meal can delay how quickly it works. You should not take sildenafil more than once a day. Sexual stimulation is needed for the medicine to work—it will not cause an automatic erection.

Avoid drinking large amounts of alcohol when taking sildenafil, as alcohol can make erectile dysfunction worse and increase the risk of side effects such as dizziness or low blood pressure. Grapefruit juice can raise sildenafil levels in the blood, so it is best to avoid it. If you experience side effects such as headache, flushing, indigestion, or a blocked nose, these are usually mild and temporary. However, if you have chest pain during sex, an erection lasting more than four hours, or sudden loss of vision or hearing, seek urgent medical help.

Timing your medicines

Levothyroxine is usually taken in the morning on an empty stomach, at least 30 minutes before breakfast, to ensure it is absorbed properly. Sildenafil is taken as needed, about one hour before sexual activity. There is no need to separate the two medicines by a specific time interval, as they do not interfere with each other's absorption. However, if you take levothyroxine in the morning and plan to take sildenafil later in the day or evening, simply follow the usual instructions for each medicine.

Other erectile dysfunction treatments and levothyroxine

If sildenafil is not suitable for you, or if you would prefer a different option, there are other PDE5 inhibitors available. Tadalafil (generic Cialis) 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) for continuous effect. Vardenafil (generic Levitra) is taken about 25 to 60 minutes before sex, and avanafil (Spedra) works faster, often within 15 to 30 minutes [4][5][6].

Like sildenafil, these medicines are generally safe to take with levothyroxine, as no major direct interaction is commonly reported. However, the same safety checks apply: the prescriber will review your full medical history and current medicines to ensure the treatment is appropriate. All PDE5 inhibitors must not be taken with nitrates, nicorandil, riociguat, or poppers, and caution is needed with alpha-blockers and certain other medicines.

If PDE5 inhibitors are not suitable or effective, other options include alprostadil cream (Vitaros) applied to the tip of the penis, or alprostadil injections (Caverject). These are prescription-only and work in a different way. There is also Eroxon, a topical gel medical device available without prescription. Your GP or an online prescriber can discuss which option is best for you, taking into account your thyroid condition and any other health issues.

When to see your GP about erectile dysfunction and thyroid health

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 underlying health problems such as heart disease, diabetes, or high blood pressure [1]. Your GP can check your overall health, review your thyroid function, and test for other conditions that might be contributing to ED.

If you are already taking levothyroxine, make sure your thyroid levels are checked regularly (usually once a year, or more often if your dose has changed). Poorly controlled hypothyroidism can worsen erectile dysfunction, so keeping your thyroid hormone levels stable is an important part of managing ED. If your thyroid levels are normal and ED persists, your GP can refer you to a specialist or discuss treatment options such as sildenafil.

You should also see your GP if you notice any new symptoms, such as chest pain, shortness of breath, or dizziness, especially during sexual activity. If you have been prescribed sildenafil and it is not working, or if you experience troublesome side effects, do not simply stop taking it or increase the dose yourself. Speak to your GP or the prescriber who issued the treatment. They can adjust the dose, try a different medicine, or investigate other causes of ED. Never take sildenafil or any other ED medicine without a prescription, and always declare all your current medicines, including levothyroxine, during the consultation.

Monitoring your thyroid and sexual health

Regular blood tests to check your thyroid-stimulating hormone (TSH) and free thyroxine (T4) levels help ensure your levothyroxine dose is correct. If your thyroid levels are too low or too high, this can affect your energy, mood, and sexual function. If you notice changes in your erectile function after starting or changing your levothyroxine dose, mention this to your GP. They can check whether your thyroid levels need adjusting. Good thyroid control, along with a healthy lifestyle, can improve your overall wellbeing and may help with erectile difficulties.

Scientific References

1. NHS. (n.d.). Erectile dysfunction (impotence). NHS website. https://www.nhs.uk/conditions/erection-problems-erectile-dysfunction/ [accessed 16 October 2026]
2. Joint Formulary Committee. (n.d.). Sildenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/sildenafil/ [accessed 16 October 2026]
3. Joint Formulary Committee. (n.d.). Tadalafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/tadalafil/ [accessed 16 October 2026]
4. Joint Formulary Committee. (n.d.). Vardenafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/vardenafil/ [accessed 16 October 2026]
5. Joint Formulary Committee. (n.d.). Avanafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/avanafil/ [accessed 16 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.

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