Sildenafil 100mg: Drug Interactions, Side Effects and How It Works

Sildenafil 100mg must not be taken with nitrate medicines such as glyceryl trinitrate or isosorbide mononitrate, with nicorandil, or with riociguat, because these combinations can cause a dangerous drop in blood pressure, alpha-blockers and some blood pressure medicines call for caution rather than automatic avoidance, and certain antibiotics and antifungals can raise sildenafil levels and may require a lower dose.

Sildenafil 100mg is the highest standard strength of one of the most widely prescribed treatments for erectile dysfunction in the UK.

For adults in the UK who are considering sildenafil 100mg or have been prescribed it and want clear, evidence-based guidance, this guide explains how it works, how it compares with branded Viagra, its contraindications and medicine interactions, key safety precautions with antibiotics and blood pressure treatments, common side effects, quality standards, regulated UK pharmacy availability, and pricing.

This is general information, not personal medical advice. Always tell your prescriber and pharmacist about every medicine, supplement and substance you use, because checking for contraindications and interactions is essential to avoid unsafe combinations and confirm that treatment is suitable for you.

What Is Sildenafil 100mg and How Does It Work?

Sildenafil belongs to a class of medicines called phosphodiesterase type 5 (PDE5) inhibitors. During sexual arousal, the body releases nitric oxide, which triggers a chain of chemical signals that relax blood vessels in the penis, allowing increased blood flow and an erection.

An enzyme called PDE5 normally breaks down part of that chemical signal, limiting the effect. Sildenafil blocks PDE5, allowing the signal to last longer and making it easier to achieve and maintain an erection during stimulation.

Importantly, sildenafil does not create arousal or desire by itself. It requires sexual stimulation to have any effect. It only works when you are sexually excited and is normally taken before sexual activity, a heavy, high fat meal can delay it's onset. Sildenafil typically starts working within 30 to 60 minutes for many people and may last for up to four hours.

Sildenafil is used to treat erectile dysfunction, and 100mg is the highest of the three standard strengths generally available (alongside 25mg and 50mg). The usual starting dose of sildenafil is 50mg, with the specific strength appropriate for you decided by your prescriber based on your response and health circumstances, not something to select for yourself.

Is Sildenafil the Same as Viagra?

Yes, in terms of the active ingredients. Sildenafil citrate is the medicine originally developed and sold under the brand name viagra, and both viagra and generic sildenafil contain the same ingredient.

Generic sildenafil, including the 100mg tablets available through EverydayMeds, contains the same active ingredient and strength and must meet the same UK regulatory standards for quality, safety and bioequivalence as the branded product.

The difference is primarily branding and price, not how the medicine works in the body.

Medicines You Must Never Combine With Sildenafil 100mg

Nitrate medicines, including glyceryl trinitrate (GTN) spray, isosorbide mononitrate and isosorbide dinitrate, used for angina and chest pain, must never be taken with sildenafil.

Nicorandil, another angina medicine, and riociguat, used to treat pulmonary arterial hypertension, carry the same serious risk, and combining sildenafil with guanylate cyclase stimulators is unsafe.

All of these relax blood vessels through a related pathway, and when combined with sildenafil can cause a potentially dangerous fall in blood pressure due to additive hypotensive effects.

Recreational drugs known as poppers (amyl nitrite, amyl nitrate, butyl nitrite) work the same way and carry the same danger.

Alpha-Blockers and Blood Pressure Medicines: Caution, Not Avoidance

Blood pressure drugs can interact with sildenafil by adding to its blood pressure-lowering effect, and alpha-blockers, such as tamsulosin, doxazosin, alfuzosin and terazosin, often prescribed for an enlarged prostate or high blood pressure, can cause symptomatic low blood pressure when combined with sildenafil, particularly on standing up. This is a genuine, well-documented interaction that calls for a cautious approach, such as a lower starting dose, rather than an outright ban.

Other blood pressure medicines, including ACE inhibitors, calcium channel blockers, beta-blockers and diuretics, may add to sildenafil's own mild blood pressure-lowering effect. For example, medicines such as Losartan are used to treat high blood pressure and, in some cases, heart failure, the combination may require clinical review, particularly where blood pressure or cardiovascular status is a concern. These combinations should be reviewed by an appropriate prescriber before sildenafil is started.

Does Sildenafil Interact With Antibiotics?

Most antibiotics have no significant interaction with sildenafil. The two worth knowing about specifically are clarithromycin and erythromycin, both macrolide antibiotics, which inhibit the liver enzyme mainly responsible for clearing sildenafil from the body.

This can meaningfully raise sildenafil levels, which is why a lower starting dose may be recommended if you need both together. If you are prescribed a different antibiotic, it is unlikely to be a specific concern. Mentioning it to your pharmacist is always a sensible check.

Antifungals and Other Medicines That Raise Sildenafil Levels

Ketoconazole and itraconazole, strong inhibitors of the same liver enzyme, can significantly increase sildenafil levels. Fluconazole may also increase sildenafil levels, although generally to a lesser extent.

HIV protease inhibitors, including ritonavir, saquinavir and indinavir, can also substantially raise sildenafil levels and typically call for a reduced dose and closer monitoring. None of these interactions automatically mean sildenafil cannot be used, they simply require your prescriber to know the full picture, including any other medications and over the counter products, so they can check for potential interactions and adjust accordingly.

Common Side Effects of Sildenafil 100mg

Most people tolerate sildenafil well. Commonly reported side effects include headache, facial flushing, indigestion, nasal congestion, and dizziness. Some people notice a temporary blue tinge to vision or increased sensitivity to light, which relates to sildenafil's mild effect on a related enzyme found in the retina, and this typically passes as the medicine clears from the body.

Extra precautions are needed in people with certain eye disorders or blood conditions. People with rare inherited retinal disorders such as retinitis pigmentosa should discuss sildenafil with a clinician before using it, and anyone with a history of NAION should avoid taking sildenafil unless specifically advised otherwise by a clinician.

Side effects tend to be more likely at higher doses, including 100mg, than at lower strengths.

Seek urgent medical attention for an erection lasting more than four hours, whether painful or not, especially if you have blood disorders such as sickle cell anemia or multiple myeloma, as in rare cases this can cause permanent damage if not treated promptly, or for any signs of a serious allergic reaction or sudden vision loss.

What Does 'BP' Mean on Sildenafil Tablets?

You may see products labelled 'sildenafil tablets BP 100mg'. Here, BP stands for British Pharmacopoeia, an official UK quality and manufacturing standard that generic medicines are tested against, not an abbreviation for blood pressure. It indicates that the tablet meets recognised UK quality standards and is a normal label on appropriately manufactured generic sildenafil products.

A Note on Unlicensed Generic Brands

You may come across sildenafil sold under names such as Aurogra, often through unregulated overseas websites. These products are not licensed for sale in the UK and are not available through regulated UK pharmacies, including EverydayMeds.

The MHRA has reported seizing large quantities of illegal erectile dysfunction medicines in recent years, some of which contained undeclared or unpredictable ingredients.

Sourcing sildenafil through a UK-regulated pharmacy following a genuine clinical assessment is the way to know what you are actually taking, and in what dose.

Quick Reference: Interaction Summary

This table summarises general information. Always check your specific combination with a pharmacist or prescriber.

Sildenafil should not be taken more than once a day.

Medical Disclaimer

This article is for general information only and does not constitute medical advice. It is not a substitute for a consultation with a qualified healthcare professional. Treatment suitability depends on an individual clinical assessment.

This medicine should only be used under the supervision of an appropriately qualified prescriber, who will review your medical history, health conditions, and other medicines to determine the appropriate strength and whether sildenafil is suitable, particularly where certain medical conditions may affect safety. Sildenafil may be suitable for some older adults following an individual clinical assessment. Results vary between individuals. If you develop severe dizziness, fainting or a prolonged erection lasting more than four hours, seek urgent medical attention, call 999 in an emergency.