Tadalafil and Tizanidine: Can You Take Them Together?

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Tadalafil and Tizanidine: Understanding the Interaction

Published: 26 October 2026

Many men prescribed tizanidine for muscle spasms wonder whether tadalafil and tizanidine can be taken together safely. Tizanidine is a muscle relaxant that lowers blood pressure, and tadalafil is a PDE5 inhibitor used to treat erectile dysfunction (ED) that also reduces blood pressure. While they are not absolutely contraindicated, combining them requires careful assessment by your prescriber because both medicines can lower blood pressure and increase the risk of dizziness or fainting.

What Is Tadalafil and How Does It Work?

Tadalafil is the active ingredient in Cialis and is one of the most widely used treatments for erectile dysfunction in the UK [4]. It belongs to a class of medicines called PDE5 inhibitors, which work by relaxing the blood vessels in the penis, allowing increased blood flow when you are sexually aroused. Tadalafil requires sexual stimulation to work—it does not cause an automatic erection.

Tadalafil is available in two forms: on-demand tablets (10mg or 20mg taken at least 30 minutes before sex, with effects lasting up to 36 hours) and daily low-dose tablets (2.5mg or 5mg taken once a day). The long duration of action makes tadalafil a popular choice for men who want more spontaneity [4]. Because tadalafil relaxes blood vessels throughout the body, it also causes a mild reduction in blood pressure, which is usually well tolerated but becomes important when combined with other blood-pressure-lowering medicines.

What Is Tizanidine and Why Is It Prescribed?

Tizanidine is a muscle relaxant prescribed to relieve muscle spasms and tightness, often associated with conditions such as multiple sclerosis, spinal cord injury, or chronic back pain. It works by acting on the central nervous system to reduce the signals that cause muscles to contract. Tizanidine is usually taken two or three times a day, and the dose is carefully adjusted to balance symptom relief with side effects.

One of the most common side effects of tizanidine is drowsiness, but it also lowers blood pressure—sometimes significantly. This blood-pressure-lowering effect is dose-related and can cause dizziness, light-headedness, or fainting, especially when standing up quickly. Because of this, tizanidine is started at a low dose and increased gradually. The blood-pressure effect is the key reason why your prescriber will carefully consider whether tadalafil and tizanidine can be taken together in your individual case.

How Tizanidine Affects Blood Pressure

Tizanidine reduces blood pressure by acting on alpha-2 receptors in the brain and spinal cord, which decreases the activity of the sympathetic nervous system. This can lead to a drop in blood pressure, particularly when you first start the medicine or increase the dose. The effect is most noticeable within one to two hours of taking a dose, and it can be more pronounced if you are also taking other medicines that lower blood pressure.

Can You Take Tadalafil and Tizanidine Together?

Tadalafil and tizanidine are not absolutely contraindicated, meaning they can sometimes be prescribed together, but only after a careful assessment by your doctor or prescriber. The main concern is that both medicines lower blood pressure, and taking them together can add to this effect, increasing the risk of dizziness, light-headedness, fainting, or even a dangerous drop in blood pressure. This is particularly important if you are taking higher doses of either medicine, or if you have other conditions or take other medicines that affect blood pressure.

Your prescriber will review your full medical history, current medicines, and the doses of both tadalafil and tizanidine before deciding whether the combination is safe for you. If the combination is considered appropriate, you will usually be started on the lowest effective dose of tadalafil and monitored closely for any signs of low blood pressure. You should never start tadalafil (or any ED treatment) without declaring that you are taking tizanidine, and you should never adjust the dose of either medicine on your own.

What Your Prescriber Will Check

Before prescribing tadalafil alongside tizanidine, your prescriber will check your baseline blood pressure, review any history of dizziness or fainting, and assess whether you are taking any other medicines that lower blood pressure (such as antihypertensives or alpha-blockers). They will also consider the dose and timing of your tizanidine, as the blood-pressure effect is strongest shortly after each dose. If you are stable on tizanidine and your blood pressure is well controlled, a low dose of tadalafil may be safe, but this decision must be made on an individual basis.

How the Interaction Works: Blood Pressure Effects

Both tadalafil and tizanidine cause blood vessels to relax, which lowers blood pressure. Tadalafil does this by inhibiting an enzyme called PDE5, which leads to increased levels of a substance called cyclic GMP that relaxes smooth muscle in blood vessel walls [4]. Tizanidine lowers blood pressure through a different mechanism—by reducing the activity of the sympathetic nervous system—but the end result is similar: blood vessels dilate and blood pressure falls.

When you take both medicines together, these blood-pressure-lowering effects can add up. In most people, the drop in blood pressure is mild and causes no symptoms, but in some cases it can lead to dizziness, weakness, fainting, or even a fall. The risk is higher if you stand up quickly (a phenomenon called postural hypotension), if you are dehydrated, or if you drink alcohol. This is why your prescriber will carefully weigh the benefits and risks before prescribing tadalafil if you are already taking tizanidine.

Practical Advice If You Are Taking Both Medicines

If your prescriber has approved the combination of tadalafil and tizanidine, there are several practical steps you can take to reduce the risk of side effects. First, always take the lowest effective dose of tadalafil, and consider timing your dose carefully—avoid taking tadalafil at the same time as your tizanidine dose, as this is when the blood-pressure-lowering effect of tizanidine is strongest. Stand up slowly from sitting or lying down to minimise dizziness, and avoid heavy alcohol, which can further lower blood pressure and worsen side effects.

Monitor yourself for warning signs such as light-headedness, dizziness, blurred vision, or feeling faint. If you experience any of these symptoms, sit or lie down immediately and contact your GP or prescriber for advice. Do not drive or operate machinery if you feel dizzy. Keep well hydrated, and avoid very hot baths or saunas, which can also lower blood pressure. If you are prescribed a new medicine or your tizanidine dose is changed, inform your prescriber before taking your next dose of tadalafil.

When to Seek Urgent Help

Seek urgent medical help if you experience severe dizziness, fainting, chest pain, a very slow or fast heartbeat, or difficulty breathing after taking tadalafil and tizanidine together. If you have an erection lasting more than four hours (priapism) or sudden loss of vision or hearing, call 999 or go to A&E immediately. These are rare but serious side effects that require prompt treatment.

Other Medicines That Interact with Tadalafil

Tadalafil interacts with several other medicines, some of which are absolutely contraindicated. Never take tadalafil with nitrates (such as GTN spray, isosorbide mononitrate, or isosorbide dinitrate), nicorandil, riociguat, or recreational drugs called 'poppers' (amyl or alkyl nitrites), as the combination can cause a dangerous, life-threatening drop in blood pressure [4]. Do not take tadalafil with another PDE5 inhibitor (such as sildenafil or vardenafil).

Alpha-blockers (such as doxazosin, tamsulosin, or alfuzosin, often prescribed for prostate problems) also lower blood pressure and require careful dose adjustment and monitoring if used with tadalafil [4]. Medicines that affect how the liver breaks down tadalafil—such as ritonavir, cobicistat, ketoconazole, itraconazole, or clarithromycin—can increase tadalafil levels and require dose adjustments. Always declare every medicine you are taking, including over-the-counter products and herbal remedies, during your consultation. Your prescriber and pharmacist will check for interactions before approving your treatment.

Scientific References

1. NHS. (n.d.). Erectile dysfunction (impotence). NHS website. https://www.nhs.uk/conditions/erection-problems-erectile-dysfunction/ [accessed 26 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 26 October 2026]
3. Joint Formulary Committee. (n.d.). Tadalafil. British National Formulary. NICE. https://bnf.nice.org.uk/drugs/tadalafil/ [accessed 26 October 2026]
4. Electronic Medicines Compendium. (n.d.). Summaries of Product Characteristics and Patient Information Leaflets. https://www.medicines.org.uk/emc/ [accessed 26 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.