Cialis 20 mg (Tadalafil) | Erectile Dysfunction, Dosing & Warnings

What Is Cialis 20 mg?

Cialis 20 mg is an oral film-coated tablet containing tadalafil, a phosphodiesterase type 5 (PDE5) inhibitor used to treat erectile dysfunction (ED) in adult men. Made by Eli Lilly, it is often called “the weekend pill” because a single dose can support erectile function for up to 36 hours. The 20 mg tablet is the highest on-demand strength.

Cialis is a prescription-only medicine. It helps a man achieve and maintain an erection only in response to sexual stimulation — it is not an aphrodisiac and does not work without arousal. Tadalafil is also used at lower daily doses for ED and for benign prostatic hyperplasia (BPH), but the 20 mg tablet is intended for on-demand use for erectile dysfunction.

Active Ingredient & Mechanism of Action

The active ingredient is tadalafil, a selective inhibitor of the enzyme phosphodiesterase type 5 (PDE5), which is found in the smooth muscle of the penis and blood vessels.

During sexual stimulation, nitric oxide is released and increases cyclic GMP (cGMP), relaxing the smooth muscle of the penis and increasing blood flow to produce an erection. Tadalafil blocks PDE5, the enzyme that breaks down cGMP, so cGMP levels stay higher for longer. This enhances the natural erectile response to stimulation — but sexual arousal is still required for it to work.

Approved Indications

Cialis is indicated for the treatment of erectile dysfunction in adult men — difficulty achieving or maintaining an erection sufficient for satisfactory sexual activity. Tadalafil (as Cialis) is also approved, usually at lower daily doses, for the signs and symptoms of benign prostatic hyperplasia (BPH) and for ED and BPH together. (A different brand of tadalafil, Adcirca, is used for pulmonary arterial hypertension.) The 20 mg tablet is used for on-demand treatment of ED.

Dosage & Administration

For on-demand use, the recommended starting dose is 10 mg taken before anticipated sexual activity, adjusted to 20 mg or down to 5 mg according to effect and tolerability. It should be taken at least 30 minutes before sexual activity and works for up to 36 hours; only one dose per day is allowed. It can be taken with or without food. Some men instead use a low daily dose (2.5–5 mg) for spontaneity. Your doctor sets the right regimen and dose.

How to Take Cialis

Swallow the tablet whole with water, with or without food, at least 30 minutes before sexual activity. Remember that an erection will only occur with sexual stimulation. Do not take more than one tablet in 24 hours, and do not combine it with other ED treatments. Avoid excessive alcohol, which can increase dizziness and lower blood pressure. If a dose does not work, do not take extra — speak to your doctor.

Missed Dose

Cialis is used as needed, not on a fixed daily schedule (except for the separate low-dose daily regimen), so there is usually no “missed dose.” If you are on the once-daily regimen and miss a dose, take it when you remember, but skip it if it is almost time for the next one — do not double up. Never take more than one dose in a 24-hour period.

Overdose

Overdose may cause more pronounced side effects such as headache, flushing, dizziness, indigestion, back pain, and low blood pressure. There is no specific antidote; management is supportive, and dialysis does not help remove tadalafil significantly. Seek medical advice if substantially more than the prescribed dose is taken, especially alongside nitrates or blood-pressure medicines.

Storage Conditions

Store Cialis in its original blister packaging at room temperature, protected from moisture; follow the temperature stated on the pack. Keep it out of the reach and sight of children. Do not use the tablets after the expiry date, and do not use any tablet that is damaged. Dispose of unused medicine responsibly according to local guidance.

Contraindications

Cialis is contraindicated with any form of nitrate (such as nitroglycerin for chest pain) and with guanylate cyclase stimulators like riociguat, because the combination can cause a dangerous, potentially fatal drop in blood pressure. It is also contraindicated in men with a known serious hypersensitivity to tadalafil, those for whom sexual activity is inadvisable due to heart disease, and men who have had non-arteritic anterior ischaemic optic neuropathy (NAION) or certain recent cardiovascular events.

Warnings & Precautions

Because sexual activity carries a degree of cardiac risk, men with significant heart disease should be assessed before use. Caution is needed with alpha-blockers and antihypertensives (additive blood-pressure lowering), significant liver or kidney impairment, a predisposition to priapism (sickle cell disease, myeloma, leukaemia, penile deformity), bleeding disorders, and active peptic ulcer. Rare but serious risks include sudden vision loss (NAION), sudden hearing loss, and a prolonged, painful erection (priapism).

Common & Serious Side Effects

The most common side effects are headache, indigestion, back pain, muscle aches, flushing, and nasal congestion; back and muscle pain are characteristic of tadalafil and usually settle within a day or two. Serious effects needing urgent care include an erection lasting more than 4 hours (priapism), sudden decrease or loss of vision in one or both eyes, sudden hearing loss or ringing, chest pain during sex, fainting, or signs of a severe allergic reaction.

Drug Interactions

The most dangerous interaction is with nitrates and riociguat (contraindicated). Alpha-blockers and other blood-pressure medicines add to its blood-pressure-lowering effect. Strong CYP3A4 inhibitors (ketoconazole, itraconazole, ritonavir, clarithromycin) raise tadalafil levels, while inducers (rifampicin, St John’s wort) lower them. Excess alcohol adds to dizziness and low blood pressure. Tell your doctor about all medicines before using Cialis.

Required Tests & Monitoring

Before prescribing, the doctor assesses cardiovascular fitness for sexual activity and reviews the underlying cause of ED, current medicines (especially nitrates and alpha-blockers), and blood pressure. No routine laboratory monitoring is generally required, but blood pressure and cardiovascular status may be checked in men with heart disease, and any warning symptoms (vision or hearing changes, prolonged erection) should be reported and evaluated promptly.

Diet & Lifestyle Support

Erectile dysfunction often reflects underlying health, so not smoking, limiting alcohol, regular exercise, a healthy weight, and good control of blood pressure, cholesterol, and diabetes all support both erectile function and overall health. Cialis can be taken with or without food, but heavy alcohol should be avoided. Addressing stress and relationship factors, and reviewing medicines that may contribute to ED, are also worthwhile.

Expected Timeline for Improvement

Taken on demand, Cialis begins to work in about 30 minutes and can support erectile function for up to 36 hours, giving a longer window than some other ED tablets. It only produces an erection with sexual stimulation. If the first dose is not effective, it may need dose adjustment or several attempts before judging the response; your doctor will advise on optimising treatment.

Effect on Other Medications & Conditions

As a mild vasodilator, tadalafil interacts strongly with nitrates, riociguat, and blood-pressure medicines, and its levels are changed by CYP3A4 inhibitors and inducers. Conditions such as heart disease, low or uncontrolled blood pressure, liver or kidney impairment, and disorders predisposing to priapism all affect its safe use. All conditions and concurrent medicines should be reviewed by the prescriber before starting.

When to See a Doctor

Seek emergency care immediately for an erection lasting more than 4 hours (priapism), sudden loss of vision, or sudden hearing loss, and for chest pain, severe dizziness, or fainting during or after sexual activity — and do not take nitrates for chest pain if you have recently used Cialis; tell the medical team you have taken it. Contact your doctor about persistent or troublesome side effects or if the medicine is not working.

Use in Special Populations

Cialis is for adult men only; it is not indicated for women or children. Dose adjustment or caution is needed in significant liver or kidney impairment, and it should be used carefully in older men and those with cardiovascular disease. It must be avoided by men taking nitrates or riociguat. Men should be able to tolerate the mild vasodilatory effects and the cardiac demand of sexual activity before using it.

Who Should Avoid Cialis?

Cialis must be avoided by men taking any nitrate or riociguat, those with serious hypersensitivity to tadalafil, men for whom sexual activity is inadvisable because of heart disease, and those with a history of NAION. Caution or avoidance also applies with significant hypotension, uncontrolled hypertension, recent heart attack or stroke, severe liver or kidney disease, and conditions predisposing to priapism.

Frequently Asked Questions

What is Cialis used for? Cialis (tadalafil) treats erectile dysfunction in men, helping achieve and maintain an erection in response to sexual stimulation; lower doses also treat BPH.

How long does it work? A single on-demand dose can support erectile function for up to 36 hours, which is why it is sometimes called the “weekend pill.”

Can I take it with nitrates? No. Combining Cialis with nitrates or riociguat can cause a dangerous drop in blood pressure and is strictly contraindicated.

How soon before sex should I take it? Take it at least 30 minutes before sexual activity; it works only with sexual stimulation.

How often can I take it? No more than one dose in any 24-hour period for on-demand use.

Medical Review & Author Note

This page was medically reviewed by Dr. Abdallah Fouad, Clinical Nutrition Specialist with over 25 years of experience. The content is provided for educational purposes to help patients understand how Cialis (tadalafil) is used, its benefits, and its risks. It does not replace the personalized advice of your treating physician or pharmacist.

Medical Disclaimer

Cialis is a prescription-only medicine that must be prescribed by a qualified physician, particularly because of its interaction with nitrates and heart medicines. This content is for general information only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your doctor or pharmacist before starting, changing, or stopping any medication.