Nattokinase, an enzyme derived from fermented soybeans, has gained attention for its purported cardiovascular benefits, leading some to wonder whether it might help with erectile dysfunction. Whilst the enzyme's theoretical effects on blood flow have sparked interest, it is crucial to understand that nattokinase is classified as a food supplement in the UK, not a licensed medicine. No clinical trials have examined its effects on erectile function, and the European Food Safety Authority has not authorised health claims relating to circulation. This article examines the evidence—or lack thereof—surrounding nattokinase and erectile dysfunction, alongside proven treatment options.

What Is Nattokinase and How Does It Work?

Nattokinase is a naturally occurring enzyme extracted from natto, a traditional Japanese food made from fermented soybeans. This proteolytic enzyme has attracted attention in complementary medicine circles for its purported effects on cardiovascular health, particularly its fibrinolytic properties—the ability to break down fibrin, a protein involved in blood clot formation.

The proposed mechanism of action centres on nattokinase's ability to enhance fibrinolysis and potentially improve blood viscosity. In laboratory studies, the enzyme has demonstrated the capacity to degrade fibrin directly and may stimulate the body's own plasminogen activator system, which naturally dissolves clots. Some proponents suggest these properties could theoretically improve circulation throughout the body, including to the penile tissues. However, these mechanistic claims are based largely on in vitro and limited animal studies; robust human data are lacking.

It is crucial to understand that nattokinase is classified as a food supplement in the UK, regulated under food law by the Food Standards Agency (FSA), not as a licensed medicine by the Medicines and Healthcare products Regulatory Agency (MHRA). This means it has not undergone the rigorous clinical trials required for therapeutic claims. The European Food Safety Authority (EFSA) has not authorised any health claims for nattokinase relating to cardiovascular benefits or circulation. The enzyme's bioavailability, optimal dosing, and long-term safety profile in humans remain inadequately characterised in peer-reviewed literature.

Whilst nattokinase is available over the counter in health food shops and online, there is no clinical evidence linking nattokinase supplementation to improvement in erectile dysfunction. No randomised controlled trials have examined erectile function as a primary outcome. The theoretical benefits based on improved circulation have not been validated through robust clinical studies. Patients considering this supplement should be aware of the distinction between traditional use and evidence-based medicine.

The Link Between Blood Flow and Erectile Function

Erectile function is fundamentally a vascular phenomenon, making adequate blood flow essential for achieving and maintaining an erection. The physiological process begins with sexual stimulation triggering the release of nitric oxide in the penile tissues, which causes smooth muscle relaxation in the corpora cavernosa—the erectile chambers of the penis. This relaxation allows arterial blood to flow into these spaces whilst venous outflow is restricted, creating the rigidity necessary for penetrative intercourse.

Vascular health is therefore paramount to erectile function. Conditions that compromise blood vessel integrity or blood flow—such as atherosclerosis, hypertension, diabetes mellitus, and hyperlipidaemia—are well-established risk factors for erectile dysfunction (ED). In fact, ED often serves as an early warning sign of systemic cardiovascular disease, typically manifesting several years before cardiac events in some patients. NICE Clinical Knowledge Summaries and the British Society for Sexual Medicine recommend cardiovascular risk assessment (including QRISK3 scoring) in men presenting with ED, alongside investigation for diabetes (HbA1c), lipid abnormalities, and blood pressure measurement.

The theoretical rationale for nattokinase's potential benefit rests on this vascular connection. If the enzyme genuinely improves blood viscosity and microcirculation, it might theoretically enhance penile blood flow. However, this hypothesis remains entirely speculative without clinical evidence. Erectile dysfunction involves complex interactions between vascular, neurological, hormonal, and psychological factors—simply improving general circulation does not automatically translate to improved erectile function.

Moreover, the penile vasculature has unique characteristics, including specialised endothelial function and smooth muscle responsiveness. Interventions that benefit systemic circulation do not necessarily affect penile haemodynamics in the same manner. NICE guidance emphasises that effective ED treatments must demonstrate specific improvements in erectile function through validated assessment tools such as the International Index of Erectile Function (IIEF), not merely theoretical mechanisms. Patients experiencing erectile difficulties should seek proper medical evaluation rather than relying on unproven supplements, as ED may indicate underlying cardiovascular disease or diabetes requiring investigation and management.

Safety Considerations and Potential Side Effects

Whilst nattokinase is generally considered safe when consumed as part of natto in the diet, concentrated supplemental forms carry potential risks that warrant careful consideration. The primary safety concern relates to the enzyme's fibrinolytic activity and consequent increased bleeding risk. Individuals taking anticoagulant medications (such as warfarin, apixaban, rivaroxaban, or edoxaban) or antiplatelet agents (including aspirin, clopidogrel, or dipyridamole) should avoid nattokinase supplements due to the potential for additive effects and serious haemorrhagic complications.

Patients scheduled for surgical procedures should discuss nattokinase use with their surgical and anaesthetic team. Typical advice is to discontinue herbal and food supplements at least one to two weeks preoperatively, as the enzyme may interfere with normal haemostasis and increase perioperative bleeding. Similarly, individuals with bleeding disorders, peptic ulcer disease, or a history of haemorrhagic stroke should exercise extreme caution or avoid this supplement entirely.

Other reported adverse effects include gastrointestinal disturbances (nausea, diarrhoea, abdominal discomfort), allergic reactions in individuals sensitive to soy products, and potential interactions with blood pressure medications. Theoretical hypotensive effects have been suggested, though clinical evidence is limited; patients taking antihypertensive medication should monitor their blood pressure and seek clinical advice if considering nattokinase. The lack of standardisation in commercial nattokinase preparations means that potency and purity can vary considerably between products, introducing additional uncertainty regarding safety and efficacy.

Nattokinase should be avoided during pregnancy and breastfeeding due to insufficient safety data. As nattokinase is a food supplement regulated under UK food law (FSA) rather than an MHRA-licensed medicine, it has not undergone the safety monitoring and quality assurance required for licensed medicines.

Patients should consult their GP or pharmacist before starting nattokinase, particularly if they have existing medical conditions or take regular medications. Self-medicating for erectile dysfunction with unproven supplements may delay proper diagnosis of underlying conditions—including cardiovascular disease, diabetes, or hormonal imbalances—that require specific medical management. If you experience unusual bleeding, bruising, or any concerning symptoms whilst taking nattokinase, discontinue use immediately and seek medical advice. You can report suspected side effects via the MHRA Yellow Card scheme at https://yellowcard.mhra.gov.uk.

Evidence-Based Treatments for Erectile Dysfunction

NICE guidelines provide clear, evidence-based recommendations for managing erectile dysfunction, emphasising treatments with proven efficacy and safety profiles. Phosphodiesterase type 5 (PDE5) inhibitors remain the first-line pharmacological treatment for most men with ED. These medications—including sildenafil, tadalafil, vardenafil, and avanafil—work by enhancing the natural erectile response to sexual stimulation through inhibition of PDE5, an enzyme that breaks down cyclic guanosine monophosphate (cGMP). This allows smooth muscle relaxation and increased blood flow to the penis.

PDE5 inhibitors have been extensively studied in randomised controlled trials using validated outcome measures such as the International Index of Erectile Function (IIEF), demonstrating significant improvements in erectile function across diverse patient populations, including those with diabetes, cardiovascular disease, and following prostate surgery. In the UK, sildenafil 50 mg (Viagra Connect) is available from pharmacies following a pharmacist-led assessment. Generic sildenafil is generally prescribable on the NHS; other PDE5 inhibitors may be subject to Selected List Scheme (SLS) restrictions and are often prescribed privately. Patients should be counselled that PDE5 inhibitors require sexual stimulation to be effective and that timing of administration varies between agents.

Important safety information for PDE5 inhibitors: These medications are absolutely contraindicated in men taking any form of nitrate medication (including glyceryl trinitrate, isosorbide mononitrate, or isosorbide dinitrate) or riociguat (a soluble guanylate cyclase stimulator), due to the risk of severe, potentially life-threatening hypotension. Caution is required in men taking alpha-blockers for benign prostatic hyperplasia or hypertension; dose adjustment and timing may be necessary. PDE5 inhibitors should be used with caution in men with unstable cardiovascular disease. Seek urgent medical attention if an erection lasts longer than four hours (priapism) or if you experience sudden loss of vision or hearing—these are rare but serious adverse effects requiring immediate assessment. Full prescribing information is available in the electronic Medicines Compendium (eMC) Summary of Product Characteristics for each agent.

Lifestyle modifications form an essential component of ED management and may improve erectile function independently or enhance response to pharmacological treatment. Evidence supports:

• Weight loss in overweight or obese men

• Regular physical exercise (at least 150 minutes of moderate-intensity activity weekly, per UK Chief Medical Officers' guidelines)

• Smoking cessation, as tobacco use significantly impairs vascular function

• Alcohol moderation, limiting intake to no more than 14 units per week (UK Chief Medical Officers' Low Risk Drinking Guidelines)

• Optimisation of cardiovascular risk factors, including blood pressure, lipid, and glucose management

For men who cannot use or do not respond to PDE5 inhibitors, second-line treatments include vacuum erection devices, intracavernosal injections (alprostadil), intraurethral alprostadil, or topical alprostadil cream. Psychological interventions, including cognitive behavioural therapy or psychosexual counselling, benefit men where psychological factors contribute significantly to ED.

When to seek medical help: Men experiencing erectile difficulties should consult their GP for proper assessment. This evaluation typically includes medical history, physical examination, and relevant investigations. NICE Clinical Knowledge Summaries recommend assessing cardiovascular risk (QRISK3), checking HbA1c (to screen for diabetes), fasting lipid profile, and blood pressure. Morning total testosterone (measured between 8am and 11am) should be checked and repeated if low; prolactin and thyroid function tests may be indicated in certain cases. ED may be the first manifestation of diabetes or cardiovascular disease, making medical evaluation essential rather than optional.

Referral pathways: Your GP may refer you to urology for structural abnormalities (such as Peyronie's disease), treatment failure, or complex cases; to endocrinology for confirmed hypogonadism or other hormonal disorders; or to cardiology if you have high or unstable cardiovascular risk. Your GP can discuss appropriate treatment options tailored to your individual circumstances, medical history, and preferences, ensuring both safety and efficacy. Self-treatment with unregulated supplements delays proper diagnosis and may expose you to unnecessary risks whilst potentially missing serious underlying conditions requiring specific management. Further information is available on the NHS website at https://www.nhs.uk/conditions/erection-problems-erectile-dysfunction/.

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