ED medications work by blocking an enzyme that controls blood flow to the penis. During arousal, your body releases nitric oxide, which triggers a chain reaction that relaxes smooth muscle tissue and widens blood vessels in the penis. This allows blood to flow in and produce an erection. The key molecule in that chain is called cGMP, and it’s constantly being broken down by an enzyme called PDE5. ED pills block that enzyme, letting cGMP build up and keeping blood vessels relaxed longer so an erection can form and last.
The important distinction: these medications don’t create arousal. They remove a physical barrier to erection once you’re already aroused. Without sexual stimulation, nothing happens.
The Four FDA-Approved Pills
All four oral ED medications use the same basic mechanism, but they differ in how fast they kick in and how long they last.
- Sildenafil (Viagra): Takes 30 to 60 minutes to work. Lasts 4 to 6 hours.
- Tadalafil (Cialis): Takes 30 to 45 minutes. Lasts up to 36 hours.
- Vardenafil (Levitra): Takes 30 to 60 minutes. Lasts 4 to 5 hours.
- Avanafil (Stendra): Takes 15 to 30 minutes. Lasts 4 to 6 hours.
Tadalafil stands out for its long window of action. That 36-hour duration is why it’s sometimes called “the weekend pill,” and it can also be prescribed as a low daily dose so you don’t need to plan around timing at all. Avanafil is the fastest to take effect, which matters if spontaneity is a priority. In head-to-head comparisons across clinical trials, all four show similar success rates: roughly 65 to 75% of men with general ED can maintain an erection through intercourse. Those numbers drop to around 40 to 50% in men with nerve-related causes like diabetes or prior pelvic surgery.
Why Food and Timing Matter
One of the most common reasons ED pills seem to “not work” is incorrect timing or eating the wrong meal beforehand. Sildenafil should be taken on an empty stomach. A high-fat meal delays its peak absorption by about an hour and reduces the amount of drug that reaches your bloodstream by nearly 30%. Vardenafil is similarly affected by fatty food. If you take sildenafil right after a steak dinner, it may feel like the medication failed when the real problem was absorption.
Tadalafil is the exception here. Its absorption isn’t meaningfully affected by food, which is one reason many men prefer it. For optimal results, on-demand tadalafil should be taken 60 to 120 minutes before sex, not the 30 minutes some men assume.
Cleveland Clinic guidelines emphasize that you should try a medication several times, with at least 24 hours between doses, before concluding it doesn’t work. A single unsuccessful attempt isn’t a fair test. Dose adjustments also matter. Your doctor may start you on a moderate dose and increase it if needed.
Common Side Effects
Because PDE5 inhibitors relax blood vessels throughout your body (not just in the penis), they produce predictable side effects. The most frequently reported ones in clinical studies include nasal congestion, headaches, and facial flushing. In one comparative study, about 61% of sildenafil users reported at least one side effect, compared to 48% with tadalafil and 23% with vardenafil. The specific patterns differ slightly between drugs. Sildenafil is more likely to cause a bluish tint to vision (about 9% of users). Tadalafil is more associated with back pain and muscle aches (13%). None of these effects are typically severe, and they resolve as the drug wears off.
The Nitrate Warning
There is one drug interaction that’s genuinely dangerous. If you take any form of nitrate medication for heart disease, such as nitroglycerin or isosorbide, combining it with a PDE5 inhibitor can cause a severe, potentially life-threatening drop in blood pressure. The reason is straightforward: nitrates increase the production of cGMP (the same molecule ED pills protect from breakdown). Together, they flood the system with cGMP, causing blood vessels throughout the body to relax far too much. In controlled studies, combining sildenafil with nitroglycerin caused dangerously low blood pressure (systolic below 85) in 36 to 46% of participants, compared to 6 to 24% on placebo. This combination is an absolute contraindication, meaning there’s no safe way to use them together.
Alpha-blockers, sometimes prescribed for high blood pressure or prostate enlargement, can also interact with PDE5 inhibitors and cause drops in blood pressure, though the risk is more manageable with dose adjustments.
When the Pills Don’t Work
About 25 to 35% of men with general ED don’t respond adequately to oral medications. Before assuming the drug has failed, it’s worth checking a few things. Incorrect use (wrong timing, taking with food, not enough sexual stimulation, not enough attempts) accounts for a large proportion of apparent treatment failures.
Low testosterone is another common culprit. A morning blood test showing levels below 300 ng/dL, combined with symptoms like low sex drive, fatigue, or loss of body hair, may indicate testosterone deficiency. Treating that underlying hormonal issue can improve how well ED medications work. Certain other medications can also contribute to the problem. Beta-blockers, some antidepressants, and certain blood pressure drugs are known to interfere with erectile function. Interestingly, even just knowing that a medication lists ED as a side effect can create enough anxiety to contribute to the problem.
Lifestyle factors play a larger role than many men expect. Smoking, obesity, untreated sleep apnea, and depression all increase the risk of ED and can reduce how well medications perform. Addressing these doesn’t replace medication, but it often makes medication more effective.
Alternatives Beyond Pills
For men who can’t take oral ED medications or don’t respond to them, other options exist that work through entirely different pathways. Alprostadil, available as a penile injection or a urethral suppository, bypasses the nitric oxide system altogether. It directly relaxes the smooth muscle inside the penis and widens the arteries feeding it. Blood flows in, fills the expandable spaces inside the erectile tissue, and compresses the veins that would normally drain it. This trapping mechanism produces a firm erection that doesn’t depend on the same enzyme pathway that pills target.
Penile injections sound intimidating, but the needle is very small and most men report minimal discomfort after the first use. The advantage is reliability: they work even in men with significant nerve damage. Vacuum erection devices and surgical implants are additional options that a urologist can discuss based on the specific cause and severity of ED.