Sildenafil and Tizanidine: Can They Be Taken Together?

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At a glance

What is sildenafil and how does it work?

Sildenafil is a medicine used to treat erectile dysfunction (ED) in men [1]. It belongs to a group of medicines called PDE5 inhibitors, which work by relaxing blood vessels in the penis, allowing more blood to flow in when you are sexually aroused [3].

Sildenafil is available in 25mg, 50mg and 100mg tablets. You usually take it about one hour before sexual activity, and it can remain effective for up to four hours [3]. The medicine only works when you are sexually stimulated—it does not cause an automatic erection.

Common brand names include Viagra (the original branded version) and various generic versions. Sildenafil can also have a mild blood-pressure-lowering effect, which is why your prescriber needs to know about all your other medicines and health conditions before prescribing it [3].

What is tizanidine and why is it prescribed?

Tizanidine is a muscle relaxant medicine used to treat muscle spasms and spasticity, often caused by conditions such as multiple sclerosis, spinal cord injury, or stroke. It works by acting on the central nervous system to reduce muscle tone and relieve painful spasms.

Tizanidine is usually taken as tablets (2mg or 4mg) and can cause drowsiness, dry mouth, and dizziness. One of its most important effects is that it can lower blood pressure, particularly when you first start taking it or when the dose is increased.

Because tizanidine affects blood pressure and can make you feel dizzy or lightheaded, it is important to tell your prescriber if you are taking any other medicines that also lower blood pressure, including treatments for erectile dysfunction.

Can you take sildenafil and tizanidine together?

Sildenafil and tizanidine can usually be taken together, but only under the supervision of a prescriber who knows your full medical history. Both medicines can lower blood pressure, so taking them at the same time may increase the risk of dizziness, lightheadedness, or fainting, especially when you stand up quickly.

This interaction is not an absolute contraindication (unlike the dangerous combination of sildenafil with nitrates, which must never be taken together). However, your prescriber will need to assess whether the combination is safe for you, taking into account your current blood pressure, other medicines, and any heart or circulation problems [3].

If your prescriber decides that sildenafil is suitable for you while you are taking tizanidine, they may recommend starting on a lower dose of sildenafil and monitoring how you feel. You should avoid sudden changes in posture (such as standing up quickly) and be cautious if you drink alcohol, as this can add to the blood-pressure-lowering effect.

How the interaction works

Both sildenafil and tizanidine cause blood vessels to relax, which lowers blood pressure. When taken together, these effects can add up, leading to a greater drop in blood pressure than either medicine would cause on its own. This is known as an additive effect.

The risk is highest when you first start treatment, change doses, or take the medicines close together in time. Symptoms of low blood pressure include feeling dizzy, faint, or unsteady, particularly when standing up. If you experience these symptoms, sit or lie down until you feel better and contact your GP or pharmacist for advice.

What you need to know

• Sildenafil and tizanidine can usually be taken together but both lower blood pressure, so prescriber assessment is essential.
• Your prescriber may start you on a lower sildenafil dose and advise spacing the medicines out during the day.
• Stand up slowly and limit alcohol to reduce the risk of dizziness or fainting.
• Report severe dizziness, chest pain during sex, or an erection lasting over four hours immediately.

What your prescriber will check before prescribing sildenafil

When you request sildenafil through an online consultation at Cured Pharmacy, a UK-registered prescriber will review your medical history and current medicines. They will ask about your blood pressure, heart health, and any medicines you take regularly, including tizanidine [2].

Your prescriber will check whether you have any conditions that make sildenafil unsuitable, such as recent heart attack or stroke, severe heart failure, very low blood pressure, or if you are taking nitrates or nicorandil (which must never be combined with sildenafil) [3].

If you are taking tizanidine or other blood-pressure-lowering medicines, your prescriber may recommend starting on the lowest dose of sildenafil (25mg) to see how you tolerate it. They will also advise you on timing—for example, taking sildenafil at a different time of day from tizanidine if possible—and what warning signs to watch for.

Declaring all your medicines

It is essential to list every medicine you take during your consultation, including prescription medicines, over-the-counter treatments, herbal remedies, and supplements. Even medicines that seem unrelated to erectile dysfunction can interact with sildenafil.

If you start a new medicine after being prescribed sildenafil, tell your GP or pharmacist that you are taking an ED treatment. Similarly, if your tizanidine dose changes, let your prescriber know so they can review whether your sildenafil dose is still appropriate.

Practical tips for taking sildenafil and tizanidine safely

If your prescriber approves the combination of sildenafil and tizanidine, follow their instructions carefully. Take sildenafil about one hour before sexual activity, and avoid taking it at the same time as your tizanidine dose if possible—spacing them out may reduce the risk of a sudden drop in blood pressure [3].

Stand up slowly from sitting or lying positions, especially in the first few hours after taking either medicine. Avoid drinking large amounts of alcohol, as this can make dizziness worse and reduce the effectiveness of sildenafil [1].

If you feel dizzy, lightheaded, or faint after taking sildenafil, do not drive or operate machinery. Sit or lie down until the symptoms pass, and contact your GP or pharmacist if the symptoms are severe or do not improve. Do not take more sildenafil than prescribed, and never take it more than once in 24 hours [3].

When to get urgent help

• Chest pain during or after sexual activity—call 999 and tell paramedics you have taken sildenafil
• An erection lasting more than 4 hours (priapism)—go to A&E immediately
• Sudden loss of vision or hearing—stop sildenafil and seek emergency help
• Severe dizziness, fainting, or very fast/irregular heartbeat—contact your GP urgently or call 111

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

When to seek urgent medical help

Most men tolerate sildenafil well, even when taking other medicines such as tizanidine, but you should be aware of warning signs that need urgent attention. If you develop chest pain during or after sexual activity, stop and seek immediate medical help—do not take more sildenafil or use a GTN spray without telling the paramedics or doctors that you have taken sildenafil [1][3].

An erection lasting more than four hours (priapism) is a medical emergency and requires immediate treatment to prevent permanent damage. Sudden loss of vision in one or both eyes, or sudden decrease or loss of hearing, are rare but serious side effects—stop taking sildenafil and go to A&E straight away [3].

If you experience severe dizziness, fainting, or a very fast or irregular heartbeat, seek same-day medical advice. These symptoms may indicate that your blood pressure has dropped too low. Your GP or pharmacist can review your medicines and adjust doses if needed.

Reporting side effects

If you experience any side effects while taking sildenafil, whether or not they seem related to tizanidine, you can report them via the Yellow Card Scheme at yellowcard.mhra.gov.uk or by searching for 'Yellow Card' in the Google Play or Apple App Store. Reporting helps improve the safety of medicines for everyone.