Cialis (tadalafil) works by blocking an enzyme called PDE5, which allows blood vessels in the penis to relax and fill with blood during arousal. What sets it apart from similar medications is its unusually long window of effectiveness, up to 36 hours after a single dose. Here’s what’s actually happening in your body when you take it.
The Biological Chain Reaction
An erection starts with a signaling molecule called nitric oxide. When you’re sexually aroused, nerves and blood vessel linings in the penis release nitric oxide, which triggers the production of a second messenger called cGMP. This cGMP is the key player: it causes the smooth muscle tissue inside the penis to relax, opening up the blood vessels so blood can rush in and produce an erection.
Your body naturally breaks down cGMP using an enzyme called PDE5. That’s part of the normal cycle. The erection eventually subsides as PDE5 chews through the available cGMP. Cialis blocks PDE5 from doing its job, so cGMP levels stay elevated longer. More cGMP means more muscle relaxation, better blood flow, and a firmer, more sustained erection.
One important detail: Cialis doesn’t create an erection on its own. It amplifies the process that sexual arousal already starts. Without that initial release of nitric oxide, the drug has nothing to work with. You still need to be aroused for it to take effect.
Why It Lasts Up to 36 Hours
Cialis is often called “the weekend pill” because its effects can persist far longer than other erectile dysfunction medications. In FDA-reviewed clinical trials, Cialis improved erectile function compared to placebo for up to 36 hours after a single dose. At the 36-hour mark, 64% of men taking the 20 mg dose reported at least one successful attempt at intercourse, compared to 37% on placebo.
This long duration comes down to the drug’s half-life, the time it takes your body to clear half the active compound from your bloodstream. Tadalafil’s half-life is roughly 17.5 hours, significantly longer than the 4 to 5 hours typical of sildenafil (Viagra). That means meaningful levels of the drug remain in your system well into the next day, giving you a much wider window without needing to time a dose precisely before sex.
Two Ways to Take It
Cialis comes in two distinct dosing strategies, and which one you use changes the experience considerably.
The on-demand approach uses a higher dose (typically 10 mg or 20 mg) taken before anticipated sexual activity. This gives you that 36-hour window but requires some planning. The daily approach uses a much lower dose (2.5 mg or 5 mg) taken at the same time every day, regardless of whether you plan to have sex. With daily dosing, the drug reaches a steady level in your bloodstream, so you don’t need to think about timing at all. Daily dosing is also the version prescribed for an enlarged prostate (more on that below).
The American Urological Association lists all PDE5 inhibitors, including Cialis, as a first-line treatment for erectile dysfunction. Their guidelines emphasize that treatment choice should be based on shared decision-making between patient and provider, not a rigid progression from least to most invasive options.
It Also Treats Enlarged Prostate Symptoms
Cialis is the only PDE5 inhibitor approved for treating the urinary symptoms of benign prostatic hyperplasia (BPH), the noncancerous prostate enlargement that commonly affects men as they age. Symptoms include frequent urination, weak stream, and difficulty starting or stopping urination.
The mechanism is related but not identical to what happens with erections. PDE5 is expressed in the blood vessels of the prostate, bladder, and surrounding pelvic tissue. By blocking PDE5 in these areas, Cialis relaxes smooth muscle in the prostate and bladder neck, reducing the physical obstruction that makes urination difficult. There’s also evidence that it improves blood flow and oxygen delivery to prostate tissue and reduces certain inflammatory markers, which may contribute to symptom relief over time. The daily 5 mg dose is what’s used for this purpose, and it can address both ED and urinary symptoms simultaneously in men who have both conditions.
Common Side Effects
In long-term clinical trials, the most frequently reported side effects were headache (about 16% of users), indigestion (12%), upper respiratory symptoms like a stuffy nose (11%), and back pain (8%). The back pain and occasional muscle aches are somewhat unique to Cialis compared to other drugs in its class. This likely relates to the fact that tadalafil, while highly selective for PDE5, also has some activity against another enzyme called PDE11. The physiological role of PDE11 isn’t fully understood, but its presence in skeletal muscle may explain these particular side effects.
Notably, Cialis is less likely to cause the temporary blue-tinged vision sometimes reported with Viagra. That visual disturbance comes from inhibiting PDE6, an enzyme found only in the retina. Tadalafil has greater selectivity for PDE5 over PDE6 than sildenafil does, making vision changes uncommon.
The Nitrate Warning
The most serious safety concern with Cialis is its interaction with nitrate medications, commonly prescribed for chest pain (angina). Both nitrates and Cialis increase levels of the same signaling molecule (cGMP) in blood vessel walls, but nitrates do it throughout the entire cardiovascular system. Combining the two can cause a dangerous drop in blood pressure.
In controlled studies, men who took nitroglycerin within 24 hours of a Cialis dose experienced significantly more episodes of systolic blood pressure dropping below 85 mmHg compared to placebo. This interaction was not seen at 48 hours or beyond. Because of Cialis’s long half-life, the risk window extends much further than it would with shorter-acting ED drugs. If you take any form of nitrate, whether pills, patches, or sprays, Cialis is contraindicated. This also applies to recreational nitrate use (poppers).