Erectile Dysfunction: Causes, Diagnosis, and Treatment Options

What Is Erectile Dysfunction?

Erectile dysfunction is the consistent inability to achieve or maintain an erection firm enough for satisfactory sexual activity. A single episode of difficulty is not diagnostic. The clinical threshold requires symptoms persisting for at least three months, assessed against validated instruments such as the five-item International Index of Erectile Function (IIEF-5), where a score below 21 signals some degree of impairment and a score below 10 reflects severe ED. [1]

The physiological mechanism behind an erection depends on nitric oxide release from endothelial cells lining the penile vasculature. Nitric oxide activates guanylate cyclase, raises cyclic GMP, and relaxes the smooth muscle of the corpus cavernosum, allowing arterial blood to fill the sinusoidal spaces. Anything that disrupts nitric oxide bioavailability, arterial flow, venous occlusion, or neural input can cause ED. The Massachusetts Male Aging Study, which followed 1,709 men aged 40 to 70, found a 52 percent age-adjusted prevalence of some degree of ED, with complete ED tripling between ages 40 and 70, from 5 to 15 percent. [2]

What Causes Erectile Dysfunction?

The causes of ED divide into four overlapping categories: vascular, neurological, hormonal, and psychogenic. Most cases in men over 40 involve a vascular component.

Vascular causes account for the largest share of organic ED. Atherosclerosis, hypertension, dyslipidemia, and type 2 diabetes all damage endothelial function and reduce penile arterial inflow. The penile artery diameter (1 to 2 mm) is smaller than the coronary artery (3 to 4 mm), which means the same degree of plaque burden produces symptoms in the penis before it produces angina. A 2010 meta-analysis in JAMA (N=2,869 men with incident cardiovascular events) found that ED independently predicted major adverse cardiac events with a hazard ratio of 1.47 (95% CI 1.10 to 1.96). [3] Because of this, men with new-onset ED and no known cardiac disease should receive a cardiovascular risk assessment before starting treatment.

Neurological causes include diabetic peripheral neuropathy, multiple sclerosis, Parkinson's disease, spinal cord injury, and post-radical prostatectomy nerve damage. Nerve-sparing prostatectomy techniques reduce but do not eliminate this risk; recovery of erectile function may take 12 to 24 months post-operatively.

Hormonal causes include hypogonadism, hyperprolactinemia, thyroid dysfunction, and, less commonly, adrenal disorders. Low testosterone reduces both libido and the density of nitric oxide synthase in cavernosal tissue.

Psychogenic causes often co-exist with organic factors. Performance anxiety, depression, and relationship conflict can each independently trigger or perpetuate ED. The presence of normal nocturnal penile tumescence (NPT) measured by RigiScan suggests a primarily psychogenic origin.

Common medications that cause or worsen ED include selective serotonin reuptake inhibitors, beta-blockers (especially atenolol), thiazide diuretics, antipsychotics, finasteride, dutasteride, and opioid analgesics. [4] A medication review is part of every ED workup.

How Is Erectile Dysfunction Diagnosed?

A systematic approach to ED includes a detailed sexual, medical, and psychosocial history, the IIEF-5 questionnaire, physical examination, and targeted laboratory testing.

Standard first-visit labs for ED include:

• Fasting glucose or HbA1c
• Fasting lipid panel
• Complete metabolic panel
• Total testosterone (drawn 7 a.m. to 11 a.m., confirmed on a second sample if low)
• Prolactin and LH/FSH (if testosterone is low)
• TSH

The American Urological Association (AUA) 2018 ED guideline states: "Serum testosterone (morning sample) should be measured in all men presenting with erectile dysfunction." [5] That single sentence has practical weight: a 2013 population survey published in The Journal of Clinical Endocrinology and Metabolism estimated 4 to 5 million American men have symptomatic testosterone deficiency, yet fewer than 10 percent receive treatment. [6]

Specialized tests, including penile Doppler ultrasound, nocturnal penile tumescence studies, and penile arteriography, are reserved for men who fail first-line therapy or who are candidates for penile prosthesis surgery.

First-Line Treatment: PDE5 Inhibitors

Oral phosphodiesterase type 5 (PDE5) inhibitors are the established first-line pharmacological treatment for ED, recommended by the AUA, the European Association of Urology (EAU), and the American College of Physicians. They work by blocking the enzyme that degrades cyclic GMP, potentiating the nitric oxide-mediated relaxation needed for erection. They require sexual stimulation to work. [5]

Four agents hold FDA approval:

| Drug | Onset | Duration | Dose range | Food effect | |---|---|---|---|---| | Sildenafil (Viagra) | 30-60 min | 4-6 hr | 25-100 mg | High-fat meal delays absorption | | Tadalafil (Cialis) | 30-45 min | Up to 36 hr | 10-20 mg PRN or 2.5-5 mg daily | None significant | | Vardenafil (Levitra) | 25-60 min | 4-5 hr | 5-20 mg | High-fat meal reduces Cmax | | Avanafil (Stendra) | 15-30 min | 6-12 hr | 50-200 mg | None significant |

In a key 1998 randomized controlled trial published in NEJM (N=532), sildenafil 100 mg improved erections in 69 percent of attempts versus 22 percent with placebo (P<0.001). [7] Tadalafil once-daily 5 mg, studied across a pooled analysis of five trials (N=1,054), produced a mean IIEF-EF domain improvement of 6.0 versus 1.3 for placebo. [8]

PDE5 inhibitors are contraindicated with any nitrate medication (nitroglycerin, isosorbide mononitrate, isosorbide dinitrate) due to the risk of severe hypotension. They require dose reduction with strong CYP3A4 inhibitors such as ketoconazole and ritonavir.

When Testosterone Is the Problem

Low testosterone does not always cause ED, but it lowers libido reliably and may reduce the response to PDE5 inhibitors. The AUA defines hypogonadism as a total testosterone below 300 ng/dL on two morning measurements combined with symptoms. [5]

A double-blind RCT published in The Journal of Clinical Endocrinology and Metabolism (N=140 hypogonadal men with ED) found that testosterone undecanoate injections alone improved IIEF scores, but the combination of testosterone undecanoate plus tadalafil 5 mg daily produced significantly greater improvement than either agent alone, with a mean IIEF-EF score increase of 9.5 vs. 5.8 for monotherapy (P<0.05). [9] This means that men with confirmed hypogonadism and ED who show an incomplete response to a PDE5 inhibitor deserve a testosterone level check before escalating to second-line therapy.

Second-Line Treatments: Injections, Devices, and Topicals

Men who fail two properly dosed trials of two different PDE5 inhibitors, or who have contraindications, have several second-line options.

Intracavernosal injections (ICI) with alprostadil (prostaglandin E1) produce erections in 70 to 80 percent of men regardless of the underlying etiology. The MUSE intraurethral alprostadil pellet offers a less invasive alternative with roughly 40 to 65 percent efficacy. The FDA approved alprostadil injection (Caverject, Edex) in 1995 specifically for ED. Starting doses range from 2.5 mcg to 5 mcg, titrated upward under clinical supervision; erections lasting over four hours (priapism) require emergency treatment.

Vacuum erection devices (VED) use negative pressure to draw blood into the corpus cavernosum, then a constriction ring maintains the erection. Satisfactory intercourse rates of 68 to 83 percent have been reported in older literature. VEDs are low-cost, carry minimal systemic risk, and are particularly suitable for men on anticoagulants.

Low-intensity extracorporeal shockwave therapy (LI-ESWT) has attracted attention as a regenerative option. A 2017 meta-analysis in European Urology (N=833 men across 7 RCTs) found a significant improvement in IIEF-EF domain scores (mean difference 2.00 to 95% CI 0.85 to 3.15, P<0.001) compared with sham treatment in men with vasculogenic ED. [10] The EAU includes LI-ESWT as an investigational option in its 2024 guidelines. The AUA's position is more cautious, noting that long-term durability data are limited.

Third-Line Treatment: Penile Prosthesis

Inflatable penile prosthesis implantation is the definitive surgical treatment for ED refractory to all medical therapies. Satisfaction rates among both patients and partners consistently exceed 90 percent in large case series. The three-piece inflatable device (AMS 700, Coloplast Titan) is the most commonly implanted type. Device survival at 10 years is approximately 60 to 80 percent before mechanical revision is needed. Infection, the most serious complication, occurs in 1 to 3 percent of primary implants.

Lifestyle Modifications That Improve ED

Lifestyle changes have Level 1 evidence in ED management and should accompany any pharmacological treatment.

A landmark randomized trial published in JAMA (N=110 obese men with ED, mean BMI 36.9) assigned men to an intensive lifestyle intervention targeting 10 percent weight loss versus control. After two years, 31 percent of men in the intervention group recovered normal erectile function versus 5 percent in controls (P<0.001). [11] The mean IIEF score improved by 7.2 points with lifestyle change.

Regular aerobic exercise independently improves erectile function. A meta-analysis in Sexual Medicine Reviews (N=1,176 men across 10 RCTs) found that moderate-intensity aerobic exercise (40-minute sessions, four times per week for six months) improved IIEF scores by a mean of 3.85 points compared with sedentary controls. [12]

Smoking cessation is directly relevant: smokers have a 1.5-fold higher risk of moderate-to-severe ED than non-smokers, and a 2015 prospective study (N=65 smokers with vasculogenic ED) demonstrated significant improvement in IIEF scores eight weeks after cessation. [13]

Premature Ejaculation: Causes and Treatment

Premature ejaculation (PE) is the most common male sexual dysfunction worldwide, defined by the International Society for Sexual Medicine (ISSM) as ejaculation occurring within approximately one minute of penetration, on most occasions, and causing significant distress. [14] The prevalence across studies using this definition is approximately 1 to 3 percent for lifelong PE, but self-reported PE affects up to 30 percent of men.

First-line pharmacological treatment for lifelong PE is daily dapoxetine (not yet FDA-approved in the U.S.) or off-label daily use of SSRIs including paroxetine 10 to 40 mg, sertraline 25 to 200 mg, or clomipramine 10 to 50 mg. Paroxetine consistently shows the strongest ejaculation delay across head-to-head trials, increasing intravaginal ejaculatory latency time (IELT) by approximately 8.8-fold from baseline. [15] Topical anesthetics (lidocaine/prilocaine spray or cream) are effective on-demand options with minimal systemic absorption. Behavioral techniques including the squeeze technique and start-stop method provide moderate benefit but require consistent practice.

Delayed Ejaculation: An Underdiagnosed Condition

Delayed ejaculation (DE) is diagnosed when a man consistently requires an unusually prolonged time to reach orgasm and ejaculate despite adequate stimulation. Many clinicians use a threshold of greater than 25 to 30 minutes as a practical guide, though no universal time cutoff exists in DSM-5. DE affects roughly 1 to 4 percent of men presenting with sexual complaints.

Causes include SSRIs (the most common drug-related cause), alpha-blockers used for benign prostatic hyperplasia, opioids, excessive alcohol use, hypothyroidism, and diabetes with autonomic neuropathy. The treatment starts with identifying and addressing the cause. Where SSRIs are the culprit, a clinician-supervised dose reduction, switch to bupropion (which has minimal ejaculatory delay), or addition of cyproheptadine 4 to 8 mg taken two hours before sexual activity may help. Psychosexual therapy is indicated when DE is primarily psychogenic or when it co-exists with a relationship conflict.

Low Libido in Men: More Than Just Testosterone

Low sexual desire in men has multiple drivers. Testosterone deficiency is the most discussed, but depression, relationship dissatisfaction, sleep disorders, chronic pain, and medication side effects each independently reduce libido.

Testosterone's role is real but nuanced. Sexual desire typically falls when total testosterone drops below 300 ng/dL, but desire may remain impaired even after testosterone is restored if underlying depression or relationship stress is not addressed. A systematic review in Journal of Sexual Medicine (N=3,160 men across 17 RCTs) found that testosterone therapy significantly improved sexual desire scores compared with placebo (standardized mean difference 0.63 to 95% CI 0.29 to 0.97). [16] The benefit was larger in men with baseline total testosterone below 230 ng/dL.

Sleep disorders, particularly obstructive sleep apnea, suppress testosterone through nocturnal hypoxemia and sleep fragmentation. A 2016 study in Sleep Medicine found that men with severe OSA had mean testosterone levels 1.84 nmol/L lower than controls matched for age and BMI. CPAP therapy for three months restored testosterone by an average of 2.1 nmol/L. [17]

A practical clinical approach to low libido in men: check total testosterone and SHBG to calculate free testosterone, screen for depression using PHQ-9, review the full medication list, order a sleep study if the Berlin Questionnaire suggests OSA, and address relationship factors with a referral to a sex therapist when indicated. Treating only testosterone without addressing these co-factors produces incomplete results in many patients.

Peyronie's Disease: The Curved Erection

Peyronie's disease (PD) is a connective tissue disorder causing fibrous plaque formation in the tunica albuginea of the penis, leading to penile curvature, pain, and ED. Prevalence estimates range from 3 to 9 percent of adult men, though the condition is under-reported due to embarrassment. [18]

The disease progresses through an acute inflammatory phase (typically 6 to 18 months, with pain and changing curvature) and a chronic stable phase (stabilized plaque, persistent curvature, possible ED). Treatment decisions depend on phase and severity.

Conservative management during the acute phase may include traction therapy with a vacuum device or penile traction device. A 12-week penile traction protocol in a 2019 single-center study (N=74 men) produced a mean curvature reduction of 20 degrees and length increase of 1.3 cm (P<0.001). [19]

Injectable collagenase clostridium histolyticum (CCH, Xiaflex) holds FDA approval for PD in men with stable disease and a curvature of at least 30 degrees. The key IMPRESS I and II trials (combined N=832) demonstrated a 34 percent mean reduction in penile curvature versus 18 percent with placebo after four treatment cycles (P<0.001). [20] Each cycle involves two injections 24 to 72 hours apart, with penile modeling performed at follow-up.

Surgical correction, including plication, grafting, or penile prosthesis implantation, is reserved for stable disease with severe curvature (greater than 60 to 70 degrees), significant ED, or failure of conservative and injectable therapy.

ED and Cardiovascular Risk: The Princeton Consensus

The Princeton III Consensus (2012) stratified men with ED into low, intermediate, and high cardiovascular risk before initiating or resuming sexual activity or starting PDE5 inhibitor therapy. Men classified as low risk (no symptoms, well-controlled hypertension, no history of MI, or NYHA Class I heart failure) can start PDE5 inhibitor therapy without further cardiac workup. Intermediate-risk patients warrant a stress test before treatment. [21]

The Princeton III panel stated: "ED is now considered an independent risk factor for cardiovascular disease and a harbinger of future cardiac events," reflecting the shared endothelial pathophysiology of both conditions. This guidance positions ED as a metabolic screening opportunity, not just a sexual complaint.

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