Cialis is a prescription medication used primarily to treat erectile dysfunction (ED). Its active ingredient, tadalafil, works by relaxing blood vessels and increasing blood flow to the penis, making it easier to get and maintain an erection during sexual activity. What sets Cialis apart from similar medications is its long duration: it can remain effective for up to 36 hours after a single dose, earning it the nickname “the weekend pill.”
How Cialis Works
During sexual arousal, your body releases a chemical messenger called cGMP that tells the smooth muscle in the penis to relax, allowing blood to flow in and produce an erection. An enzyme called PDE5 normally breaks down cGMP after it does its job. In men with ED, this breakdown can happen too quickly or the signaling doesn’t work efficiently enough to produce a firm erection.
Cialis blocks the PDE5 enzyme, which lets cGMP build up and do its work more effectively. The result is stronger blood flow and a more reliable erection. It does not create arousal on its own. Sexual stimulation is still necessary for the medication to have any effect.
What Cialis Is Approved to Treat
The FDA has approved Cialis for three uses: erectile dysfunction, the urinary symptoms of an enlarged prostate (benign prostatic hyperplasia, or BPH), and the combination of both conditions together. Many men over 50 deal with ED and BPH at the same time, so a single daily pill can address both problems.
For BPH, Cialis works by relaxing smooth muscle in the prostate, bladder, and the blood vessels that supply them. This improves blood flow and oxygenation to those tissues, which eases symptoms like a weak urine stream, frequent urination, and the urgency to go. The same active ingredient is also sold under the brand name Adcirca for pulmonary arterial hypertension (high blood pressure in the lungs), though at a much higher dose. You should never take both Cialis and Adcirca together.
How Long It Takes and How Long It Lasts
Cialis can start working in as little as 30 minutes for some men, though most people find it reaches full effect within one to two hours. It stays active in your body far longer than similar ED medications. Clinical trials showed improved erectile function up to 36 hours after a single dose. The drug has a half-life of about 17.5 hours, meaning it takes that long for your body to clear just half of the dose.
This extended window is the main practical advantage of Cialis. Rather than timing a pill closely around sexual activity, you have a much wider window of spontaneity. For comparison, sildenafil (Viagra) typically lasts four to six hours.
Daily vs. As-Needed Dosing
Cialis comes in two dosing strategies. The as-needed approach uses a 10 mg or 20 mg tablet taken before anticipated sexual activity. You don’t need to take it every day, just when you want it to be working.
The daily approach uses a much smaller dose, starting at 2.5 mg and going up to 5 mg if needed. You take it at the same time every day regardless of whether you plan to have sex. Because the drug is always in your system at a steady level, you’re ready whenever the moment arises without planning ahead. Daily dosing is also the approach used to treat BPH symptoms, with the standard dose being 5 mg once daily.
Your doctor will typically recommend one approach over the other based on how frequently you’re sexually active, whether you also have BPH, and how you respond to the medication.
Common Side Effects
Cialis is generally well tolerated, but side effects do occur. In clinical trials of the 20 mg as-needed dose, the most common were:
- Headache: affected about 15% of men (compared to 5% on a placebo)
- Indigestion or upset stomach: about 10%
- Back pain: about 6%
Side effects tend to be milder and less frequent with the lower daily doses. At the 5 mg daily dose, headache occurred in about 4 to 6% of men and indigestion in about 2 to 5%. Back pain was reported by 2 to 3%. These effects are usually mild and go away on their own within a few hours. Nasal congestion, flushing, and muscle aches can also occur but are less common.
Dangerous Drug Interactions
The most critical safety concern with Cialis is its interaction with nitrate medications, which are commonly prescribed for chest pain (angina). Both Cialis and nitrates cause blood vessels to relax through the same chemical pathway. Taking them together can cause a dangerous, potentially life-threatening drop in blood pressure. Cialis is strictly contraindicated with all forms of nitrates, including nitroglycerin patches, sprays, and tablets. If you’ve taken Cialis, nitrates should not be used for at least 48 hours afterward.
Alpha-blockers, often prescribed for high blood pressure or BPH symptoms, also carry interaction risks. Combining them with Cialis can cause dizziness and drops in blood pressure when standing up. If you’re already on a stable alpha-blocker dose, a doctor can introduce Cialis at a low dose with monitoring. Most other blood pressure medications can be safely taken alongside Cialis without causing significant blood pressure problems.
Who Should Not Take Cialis
Beyond the nitrate restriction, Cialis is not appropriate for everyone. People with severe liver or kidney disease, those who have had a recent stroke or heart attack, or anyone with uncontrolled low blood pressure need to avoid it or use it only under close medical supervision. A rare but serious side effect called priapism (an erection lasting more than four hours) requires immediate medical attention to prevent permanent damage. Sudden changes in vision or hearing have also been reported in rare cases with all PDE5 inhibitors.
Cialis is available as a generic (tadalafil) since the brand-name patent expired, which has made it significantly more affordable. Both the brand and generic versions contain the same active ingredient and work identically.