Cialis (tadalafil) is a prescription medication used primarily to treat erectile dysfunction (ED) and the urinary symptoms of an enlarged prostate. It also treats pulmonary arterial hypertension, a condition involving high blood pressure in the lungs, under the brand name Adcirca. What sets Cialis apart from similar medications is its long duration: effects can last up to 36 hours, earning it the nickname “the weekend pill.”
Erectile Dysfunction
Cialis is best known as a treatment for erectile dysfunction. It works by relaxing smooth muscle tissue in blood vessel walls, which increases blood flow to the penis during sexual arousal. The drug doesn’t cause spontaneous erections on its own. Sexual stimulation is still needed to trigger the natural chain of events that produces an erection.
In FDA-reviewed clinical trials, men taking the standard 20 mg dose were significantly more likely to achieve and maintain erections compared to those on a placebo. In one study, 64% of men on Cialis maintained erections sufficient for intercourse, compared to 23% on placebo. In another, the success rate was 50% versus 25%. These numbers reflect real-world variability: results depend on the severity of ED, underlying health conditions, and other individual factors.
One of Cialis’s defining advantages is how long it works. Clinical data show that 61% of men on the 20 mg dose reported successful intercourse at the 24-hour mark, and 64% still reported success at the 36-hour mark. By comparison, sildenafil (Viagra) has a half-life of about 4 hours, while tadalafil’s half-life is 17.5 hours. That longer window gives couples more flexibility and less pressure around timing.
Some men in clinical trials reported effects as early as 30 minutes after taking the pill. In one study, 52% of men on the 20 mg dose achieved a successful erection within that first half hour. That said, effects can take longer to kick in for others, and food doesn’t meaningfully delay absorption the way it can with some competing medications.
Two Ways to Take It for ED
Cialis can be taken in two different ways for erectile dysfunction. The first is as-needed dosing: a 10 mg or 20 mg tablet taken roughly 30 minutes before anticipated sexual activity, no more than once per day. This approach works well for men who don’t need the medication regularly.
The second option is a low daily dose of 2.5 mg or 5 mg, taken at the same time every day regardless of when sexual activity might happen. Because the drug is always active in your system, daily dosing removes the need to plan ahead. In clinical trials, men on 5 mg daily reported successful erections 67% to 79% of the time, compared to 51% to 52% on placebo. Daily dosing also makes Cialis the only ED medication that doubles as a treatment for enlarged prostate symptoms.
Enlarged Prostate Symptoms
Cialis is FDA-approved at a 5 mg daily dose for treating the lower urinary tract symptoms caused by benign prostatic hyperplasia (BPH), a noncancerous enlargement of the prostate that becomes increasingly common after age 50. Symptoms include a weak urine stream, difficulty starting urination, frequent urges to urinate (especially at night), and the feeling that the bladder hasn’t fully emptied.
The medication relaxes smooth muscle in the prostate, bladder, and surrounding blood vessels. This improved blood flow and reduced muscle tension eases urinary symptoms in a way that clinical studies show is comparable to the symptom relief provided by alpha-blockers, the traditional first-line medications for BPH. One important distinction: Cialis improves how symptoms feel but does not significantly increase the actual rate of urine flow. It treats the discomfort of BPH more than the mechanical obstruction.
For men who have both ED and BPH, a single daily 5 mg dose can address both conditions. In a trial of men with both conditions, 72% maintained erections adequate for intercourse on the 5 mg dose, compared to 48% on placebo.
Pulmonary Arterial Hypertension
Under the brand name Adcirca, tadalafil is prescribed at a higher dose (40 mg once daily) to treat pulmonary arterial hypertension (PAH). This is a serious condition where blood pressure in the arteries supplying the lungs is abnormally high, forcing the heart to work harder and limiting physical activity. The same mechanism that relaxes blood vessels in the pelvis also relaxes the arteries in the lungs, reducing the strain on the heart. Adcirca is approved for both adults and children from age 2, covering patients with mild to moderate limitations in physical activity.
How It Works in the Body
All of Cialis’s uses stem from the same basic mechanism. During normal function, the body produces nitric oxide to signal blood vessels to relax. Nitric oxide triggers the production of a molecule called cGMP, which does the actual work of loosening smooth muscle in vessel walls. An enzyme called PDE5 constantly breaks down cGMP, keeping its levels in check. Cialis blocks PDE5, allowing cGMP to accumulate and blood vessels to stay relaxed longer. This same enzyme is found in the penis, the prostate, the bladder, and the lungs, which is why one drug can treat such different conditions.
Side Effects
The most common side effects are headache, indigestion, back pain, muscle aches, flushing, and nasal congestion. These tend to be mild and temporary, often fading as the body adjusts to the medication. Back pain and muscle aches are more unique to Cialis compared to other ED medications and typically show up 12 to 24 hours after taking a dose.
Less commonly, some men experience dizziness or a slight drop in blood pressure. A rare but serious side effect is a prolonged erection lasting more than four hours, which requires immediate medical attention to prevent tissue damage. Sudden changes in vision or hearing have been reported in very rare cases.
Dangerous Interactions With Nitrates
The most critical safety concern with Cialis is its interaction with nitrate medications, which are commonly prescribed for chest pain and heart conditions. Both Cialis and nitrates increase cGMP levels through different pathways. Taking them together causes an excessive buildup that can trigger a severe, potentially life-threatening drop in blood pressure. This applies to all nitrate medications, including nitroglycerin and isosorbide mononitrate.
This interaction also extends to recreational drugs known as “poppers” (amyl nitrate or nitrite), which work through the same pathway and can cause the same dangerous blood pressure crash when combined with Cialis.
Alcohol and Grapefruit
Because Cialis already lowers blood pressure slightly, alcohol can amplify that effect. Keeping alcohol intake moderate (no more than three or four drinks in a short period) reduces the risk of dizziness, lightheadedness, and flushing. Grapefruit juice can increase blood levels of tadalafil by interfering with how the body breaks it down, so large quantities are best avoided while using the medication.