Sildenafil is best known as the active ingredient in Viagra for erectile dysfunction, but it carries two distinct FDA approvals and a surprisingly long list of off-label and investigational uses. Marketed under the brand name Revatio for pulmonary arterial hypertension, it has also been studied for conditions ranging from digital ulcers in autoimmune disease to Alzheimer’s prevention. The drug’s journey from a failed angina medication to one of the most versatile molecules in modern pharmacology is one of the more unusual stories in drug development, and the range of conditions it may treat keeps expanding.
How Sildenafil Actually Works
Sildenafil blocks an enzyme called phosphodiesterase type 5 (PDE5). That enzyme normally breaks down a signaling molecule called cyclic GMP, which tells smooth muscle cells lining blood vessels to relax. By slowing the breakdown of cyclic GMP, sildenafil keeps blood vessels dilated longer. This is a simple effect with broad consequences: anywhere the body relies on PDE5 to regulate blood flow, sildenafil can shift the balance toward more relaxation and better circulation.1PubMed. The effect of sildenafil on corpus cavernosal smooth muscle relaxation and cyclic GMP formation in the diabetic rabbit
PDE5 is concentrated in certain tissues, which is why the drug’s effects show up in specific places rather than everywhere at once. The penile blood vessels are rich in PDE5, which is why erectile dysfunction was the first clinical hit. But PDE5 is also abundant in the blood vessels of the lungs, and it appears in cardiac tissue, the walls of peripheral arteries, and even certain parts of the brain. Each approved or investigational use of sildenafil traces back to this same enzyme in a different tissue.
Erectile Dysfunction
The original and still most common use. When the landmark trial was published in the New England Journal of Medicine in 1998, men taking 100 mg of sildenafil doubled their erection scores compared to baseline, and about 69% of intercourse attempts were successful versus 22% on placebo.2PubMed. Oral Sildenafil in the Treatment of Erectile Dysfunction Those numbers held up across a wide range of causes, including diabetes, spinal cord injury, and recovery after prostate surgery, all populations traditionally considered difficult to treat.3PubMed Central. Sildenafil in the treatment of erectile dysfunction: an overview of the clinical evidence
Onset can be as fast as 14 minutes for some men, with effects lasting up to 12 hours, though most people experience a practical window of four to six hours.4PubMed Central. Sildenafil in the treatment of erectile dysfunction: an overview of the clinical evidence An important detail that sometimes gets lost: sildenafil does not create erections on its own. It amplifies the body’s normal arousal response by keeping cyclic GMP around longer. Without sexual stimulation, nothing happens. This is different from drugs that directly inject a chemical signal into the tissue, which can produce erections regardless of arousal.
The composition-of-matter patent expired in late 2017 in the United States, and generic sildenafil became available almost immediately. Within the first year, average wholesale prices dropped by more than 90%, and overall use rose substantially as out-of-pocket costs fell.5PubMed Central. The Economics of Viagra Revisited: The Price Is Right. What had been an expensive brand-name medication became one of the cheapest prescriptions in the pharmacy.
Pulmonary Arterial Hypertension
Sildenafil’s second approved indication is pulmonary arterial hypertension (PAH), a condition where the blood vessels in the lungs narrow and stiffen, forcing the right side of the heart to work dangerously hard. The drug is sold under the name Revatio for this purpose, typically at a much lower dose (20 mg three times daily) than the erectile dysfunction dose.
The pivotal trial enrolled patients across multiple functional classes and tested three dose levels. All doses lowered mean pulmonary artery pressure, improved patients’ ability to exercise, and improved their functional classification on a standard severity scale.6PubMed. Sildenafil citrate therapy for pulmonary arterial hypertension Smaller studies and case series leading up to that trial had already shown improvements in pulmonary vascular resistance and cardiac output.7PubMed Central. Sildenafil in the treatment of pulmonary hypertension For patients with PAH, sildenafil can be the difference between being housebound and being able to walk a meaningful distance. It is now a standard part of the treatment toolkit for this disease, often used alongside other pulmonary vasodilators.
Digital Ulcers in Systemic Sclerosis
Systemic sclerosis (scleroderma) causes the small blood vessels in the fingers to constrict severely, sometimes leading to painful digital ulcers that heal slowly or not at all. Because sildenafil opens up small blood vessels, clinicians began trying it for these ulcers off-label. A pilot study found that patients starting with an average of about three ulcers per person saw that number drop significantly, with pain and Raynaud’s symptoms also improving.8PubMed Central. Effect of sildenafil on digital ulcers in systemic sclerosis: analysis from a single centre pilot study
A larger placebo-controlled trial (the SEDUCE study) confirmed the benefit. By eight weeks, the healing rate for ulcers present at baseline was roughly 80% higher in the sildenafil group than in the placebo group, and the advantage persisted at twelve weeks.9Annals of the Rheumatic Diseases. Efficacy of sildenafil on ischaemic digital ulcer healing in systemic sclerosis: the placebo-controlled SEDUCE study This is not an FDA-approved use, but rheumatologists prescribe sildenafil for digital ulcers with some regularity, particularly when standard therapies have failed.
Women’s Sexual Dysfunction
Sildenafil’s effect on sexual function is not limited to men. The mechanism of cyclic GMP-driven blood flow applies to genital tissue in women as well, and early reports suggested it could help women experiencing sexual dysfunction caused by antidepressants, particularly selective serotonin reuptake inhibitors (SSRIs). An initial open-label series of nine women found that all reported significant reversal of SSRI-induced anorgasmia or delayed orgasm, usually with the first 50 mg dose.10PubMed. Sildenafil for women patients with antidepressant-induced sexual dysfunction
A randomized controlled trial followed up on those early findings. Premenopausal women taking antidepressants were randomized to sildenafil or placebo, and the sildenafil group showed a statistically significant improvement in a global sexual-function score compared with placebo.11JAMA. Sildenafil Treatment of Women With Antidepressant-Associated Sexual Dysfunction: A Randomized Controlled Trial This is a narrower indication than general female sexual dysfunction. Trials in women without antidepressant-related problems have been mixed, and sildenafil has never been approved for women. But for the specific problem of SSRI-induced sexual side effects, the evidence is reasonably encouraging.
Exercise at High Altitude
At high altitude, low oxygen levels cause the blood vessels in the lungs to constrict, raising pulmonary artery pressure and limiting how much blood the heart can push through. Because sildenafil relaxes those same vessels, researchers tested whether it could improve exercise performance in hypoxic conditions. The answer is a qualified yes.
In a crossover trial at simulated altitude and at Mount Everest base camp, sildenafil increased maximum workload by roughly a third compared to placebo, boosted cardiac output, and lowered pulmonary artery pressure during both rest and exercise.12PubMed. Sildenafil increased exercise capacity during hypoxia at low altitudes and at Mount Everest base camp: a randomized, double-blind, placebo-controlled crossover trial A separate study found that sildenafil lowered time-trial performance by 15% (meaning faster cycling), though the benefit was not uniform. Some subjects responded dramatically, improving by nearly 40%, while others showed almost no change.13PubMed. Sildenafil improves cardiac output and exercise performance during acute hypoxia, but not normoxia A meta-analysis confirmed that at altitudes above 5,000 meters, sildenafil increased cardiac output at rest compared to placebo, though the effect during exercise did not reach statistical significance across studies.14PubMed Central. Efficacy of Sildenafil on healthy humans in high‑altitude hypoxia at rest and during exercise: A meta‑analysis
The important caveat: these benefits appear under hypoxic conditions. At sea level, sildenafil does not improve exercise capacity in healthy people. The drug offsets a specific bottleneck caused by altitude-induced pulmonary vasoconstriction, and when that bottleneck is absent, there is nothing to offset. Also, one randomized trial testing sildenafil for prevention of high-altitude pulmonary hypertension in healthy lowlanders found it did not reduce pulmonary artery pressure and actually worsened acute mountain sickness symptoms on the second day at 5,200 meters.15PubMed. Sildenafil citrate for the prevention of high altitude hypoxic pulmonary hypertension: double blind, randomized, placebo-controlled trial So sildenafil is not a universal altitude pill. It can help exercise performance at extreme elevation for some people, but it does not reliably prevent altitude sickness, and routine prophylactic use is not supported.
Investigational Cardiovascular Uses
Beyond pulmonary hypertension, researchers have explored sildenafil’s effects on the heart itself. Animal studies have been the main source of evidence so far, but they paint a surprisingly hopeful picture. In a mouse model of heart attack, sildenafil reduced the amount of dead heart tissue by nearly half compared to saline, preserved the heart’s pumping function over 28 days, and dramatically improved survival from 36% to 93%.16American Journal of Physiology-Heart and Circulatory Physiology. Sildenafil (Viagra) attenuates ischemic cardiomyopathy and improves left ventricular function in mice In a model of high-blood-pressure-driven heart failure, sildenafil reduced cardiac hypertrophy, fibrosis, and inflammation while improving both the heart’s ability to contract and its ability to relax between beats.17PubMed. Selective PDE5A inhibition with sildenafil rescues left ventricular dysfunction, inflammatory immune response and cardiac remodeling in angiotensin II-induced heart failure in vivo
A separate study in obese rats with heart failure with preserved ejection fraction (HFpEF), one of the most frustrating conditions in cardiology because so few drugs help, found that chronic sildenafil treatment lowered blood pressure, reduced left ventricular stiffness, and modestly improved exercise tolerance.18PubMed. Chronic Sildenafil Therapy in the ZSF1 Obese Rat Model of Metabolic Syndrome and Heart Failure With Preserved Ejection Fraction These are animal data, and translating them to humans requires clinical trials that are still ongoing or in early stages. But the consistent direction of benefit across different types of heart injury is why the drug keeps attracting cardiovascular researchers.
Alzheimer’s Disease Research
This is arguably the most surprising frontier for sildenafil. PDE5 appears to be overactive in the brains of people with Alzheimer’s disease, meaning that cyclic GMP signaling, which supports blood flow, nerve-cell communication, and clean-up of toxic proteins, is weakened. Sildenafil theoretically addresses this by restoring cyclic GMP levels, and laboratory work suggests it may suppress production of amyloid-beta (a hallmark Alzheimer’s protein), boost mitochondrial function, and encourage growth of new neurons.19PubMed Central. Sildenafil for the Treatment of Alzheimer’s Disease: A Systematic Review
Real-world patient data has reinforced this interest. A study using large insurance databases found that sildenafil users had roughly 30-54% lower rates of Alzheimer’s diagnosis compared to users of other medications, depending on the comparator drug and the database.20PubMed Central. Sildenafil as a Candidate Drug for Alzheimer’s Disease: Real-World Patient Data Observation and Mechanistic Observations from Patient-Induced Pluripotent Stem Cell-Derived Neurons The same researchers showed that sildenafil reduced tau hyperphosphorylation, another Alzheimer’s hallmark, in patient-derived neurons in the lab. A meta-analysis pooling the available observational studies estimated that sildenafil use was associated with roughly half the risk of developing Alzheimer’s compared to non-use.21PubMed Central. Sildenafil and risk of Alzheimer disease: a systematic review and meta-analysis
The reason this remains investigational rather than a recommendation is that all of these data are observational. People who take sildenafil differ from people who do not in ways that could explain the lower Alzheimer’s rates: they tend to be healthier, more sexually active, and more engaged with the healthcare system. Randomized trials are needed before anyone should take sildenafil to prevent dementia. But the combination of a plausible mechanism, supportive lab data, and consistent epidemiological signals is why multiple clinical trials are now underway.
Pregnancy Complications
Sildenafil has been investigated for fetal growth restriction and preeclampsia, two dangerous pregnancy conditions linked to poor blood flow through the placenta. The theory is straightforward: if sildenafil dilates the uterine arteries, it might improve delivery of oxygen and nutrients to the fetus. A preclinical meta-analysis across animal models found that sildenafil did increase fetal growth during complicated pregnancies compared to healthy ones and lowered maternal blood pressure significantly, but only when blood pressure was already elevated.22PubMed. Sildenafil During Pregnancy: A Preclinical Meta-Analysis on Fetal Growth and Maternal Blood Pressure Small human studies have explored whether sildenafil can improve Doppler blood-flow measurements in growth-restricted pregnancies.23PubMed Central. Sildenafil Citrate in Fetal Growth Restriction
However, a major Dutch clinical trial (STRIDER) was halted early after an interim analysis found more newborn deaths in the sildenafil group, possibly related to a harmful blood-pressure effect in the newborn’s lungs. This was a sobering outcome that has significantly cooled enthusiasm for this particular use. Sildenafil is not recommended for pregnant women outside of carefully monitored clinical trials, and the future of this research direction is uncertain.
Safety, Side Effects, and Drug Interactions
The most common side effects are predictable consequences of vasodilation: flushing, headache, nasal congestion, and indigestion. These tend to be mild and dose-related. The serious safety concern that matters most in practice is the interaction with nitrate medications, such as nitroglycerin or isosorbide dinitrate, commonly prescribed for angina. Both sildenafil and nitrates work through the same cyclic GMP pathway, and combining them can cause a severe, potentially fatal drop in blood pressure.24PubMed. Potentiation of sildenafil-induced hypotension is minimal with nitrates generating a radical intermediate This combination is absolutely contraindicated, and emergency physicians are trained to ask about recent sildenafil use before administering nitrates for chest pain.25PubMed. The clinical safety of viagra
Alpha-blockers, often prescribed for high blood pressure or prostate enlargement, can also cause a significant additive blood-pressure drop when taken with sildenafil. Current prescribing guidance recommends caution, dose adjustment, and timing separation when combining these drugs.26PubMed. Pharmacology and drug interaction effects of the phosphodiesterase 5 inhibitors: focus on alpha-blocker interactions Sildenafil is also processed primarily by a liver enzyme called CYP3A4, so drugs that inhibit this enzyme (including some antibiotics and antifungals) can raise sildenafil levels in the blood and intensify both the effects and the side effects.27PubMed. The effect of ciprofloxacin and clarithromycin on sildenafil oral bioavailability in human volunteers
A rarer concern involves a type of sudden vision loss called nonarteritic anterior ischemic optic neuropathy (NAION). Case reports have described it developing within minutes to hours after taking sildenafil, particularly in people with a structurally small optic nerve head.28PubMed. Sildenafil-associated nonarteritic anterior ischemic optic neuropathy However, a large analysis looking at over 13,000 men in clinical trials and more than 35,000 patient-years of observation estimated the rate of NAION at about 2.8 cases per 100,000 patient-years of exposure, which is similar to the background rate in the general population of men over 50.29PubMed Central. Sildenafil citrate use and the incidence of nonarteritic anterior ischemic optic neuropathy The relationship remains uncertain, but if you experience sudden vision loss after taking sildenafil, it warrants immediate medical attention.
The Counterfeit Problem
Sildenafil has been one of the most counterfeited drugs in the world, largely because many people buy it online without a prescription out of embarrassment. In one study, 77% of Viagra tablets ordered through internet pharmacies were counterfeit.30PubMed. Internet-ordered viagra (sildenafil citrate) is rarely genuine The fakes contained anywhere from 30% to 50% of the active ingredient they claimed, and none came with proper safety information.
Analysis of counterfeit PDE5 inhibitors more broadly found that active ingredient levels ranged from 0% to more than 200% of what was on the label. Only about 10% of samples fell within an acceptable range of the stated dose. Contaminants found in counterfeits have included talcum powder, commercial paint, and printer ink.31PubMed Central. Counterfeit phosphodiesterase type 5 inhibitors pose significant safety risks Beyond the direct dangers of unknown ingredients, counterfeit pills bypass the healthcare system entirely, meaning users get no screening for cardiovascular risk factors, no warning about nitrate interactions, and no dosing guidance.32PubMed Central. The dangers of sexual enhancement supplements and counterfeit drugs to “treat” erectile dysfunction The availability of cheap generic sildenafil since 2017 has likely reduced the economic incentive for counterfeiting, but online sources remain risky.
Cancer Immunotherapy Enhancement
An entirely different line of research has explored sildenafil as a way to make cancer immunotherapy work better. In lab experiments with lung and ovarian cancer cells, combining sildenafil with the chemotherapy drug pemetrexed reduced expression of PD-L1, a protein tumors use to hide from the immune system, and increased expression of molecules that help immune cells recognize tumor cells. In mice with lung cancer, the combination enhanced the anti-tumor effects of checkpoint inhibitor antibodies and increased infiltration of the tumors by activated immune cells including T cells, NK cells, and macrophages.33PubMed Central. pemetrexed + sildenafil, via autophagy-dependent HDAC downregulation, enhances the immunotherapy response of NSCLC cells This is early-stage research, entirely preclinical, but it represents a growing interest in repurposing PDE5 inhibitors as immune modulators rather than purely vascular drugs.
Veterinary Uses
Sildenafil has crossed into veterinary medicine as well, primarily for dogs with pulmonary hypertension. Dogs with degenerative mitral valve disease frequently develop secondary pulmonary hypertension, and a controlled trial showed that sildenafil reduced estimated systolic pulmonary artery pressure by about 7 mmHg within a week and improved overall clinical scores compared to placebo.34PubMed Central. Short-term effects of sildenafil in the treatment of dogs with pulmonary hypertension secondary to degenerative mitral valve disease
A more unexpected veterinary application involves megaesophagus, a condition where the esophagus loses its ability to push food into the stomach, causing chronic regurgitation. A randomized trial in dogs with congenital megaesophagus found that sildenafil significantly reduced regurgitation episodes, from an average of about 2.7 per period down to less than 1, and improved weight gain compared to controls. The drug appeared to work by relaxing the lower esophageal sphincter, a muscular ring whose excessive tone may contribute to the problem.35PubMed. Sildenafil improves clinical signs and radiographic features in dogs with congenital idiopathic megaoesophagus: a randomised controlled trial For a condition with very few effective treatments, this represented a meaningful advance.
From Failed Angina Drug to Everywhere
Sildenafil was originally developed in the late 1980s as a treatment for angina, chest pain caused by narrowed coronary arteries. It did relax blood vessels, but its effects on coronary blood flow were disappointing. During clinical trials, male participants started reporting an unexpected side effect: improved erections. Pfizer pivoted, and the drug was approved for erectile dysfunction in 1998.36PubMed Central. Sildenafil: from angina to erectile dysfunction to pulmonary hypertension and beyond The discovery has been called one of the most serendipitous in pharmaceutical history.37Sexual Medicine Reviews. The Serendipitous Story of Sildenafil: An Unexpected Oral Therapy for Erectile Dysfunction In less than 20 years from that original angina work, sildenafil had picked up its second approval for pulmonary hypertension and spawned active research programs across half a dozen other medical fields. Few drugs have had such a productive second act after failing at what they were designed to do.