Tadalafil was first approved by the U.S. Food and Drug Administration (FDA) in November 2003, specifically on November 21, 2003, for the treatment of erectile dysfunction and was marketed under the brand name Cialis. It belongs to the phosphodiesterase type 5 (PDE5) inhibitor class and works by increasing blood flow to the penis during sexual stimulation through enhanced nitric oxide–cGMP signaling and smooth muscle relaxation. Following its initial approval, tadalafil’s medical use was expanded in later years to include benign prostatic hyperplasia (BPH) and pulmonary arterial hypertension under different dosing regimens and brand formulations. Its relatively long half-life, which supports a duration of action up to 24–36 hours, distinguishes it from other drugs in its class and allows for both on-demand and once-daily dosing strategies depending on the clinical indication.
BRAND NAMES
Tadalafil is available under the brand names Cialis and Adcirca. Cialis is primarily used for erectile dysfunction and benign prostatic hyperplasia, while Adcirca is used for pulmonary arterial hypertension.
MECHANISM OF ACTION
Tadalafil works by inhibiting the enzyme phosphodiesterase type 5 (PDE5), which increases levels of cyclic guanosine monophosphate (cGMP) in smooth muscle cells. This leads to relaxation of blood vessels and improved blood flow to the penis during sexual stimulation, helping to achieve and maintain an erection.
PHARMACOKINETICS
Absorption
Tadalafil is rapidly absorbed after oral administration, with peak plasma concentrations typically reached within about 2 hours. Its absorption is not significantly affected by food, allowing it to be taken with or without meals. The drug has good oral bioavailability and demonstrates predictable pharmacokinetics across standard dosing ranges.
Distribution
Tadalafil is widely distributed in body tissues after absorption and shows a large volume of distribution, indicating extensive tissue binding beyond the bloodstream. It is highly protein bound in plasma, mainly to albumin, which helps maintain its prolonged duration of action. The drug can reach target tissues such as the smooth muscle of the corpus cavernosum, where it exerts its therapeutic effect.
Metabolism
Tadalafil is primarily metabolized in the liver by the cytochrome P450 enzyme system, mainly through CYP3A4. It is converted into inactive metabolites through processes such as demethylation and oxidation. Because of this pathway, drugs that strongly inhibit or induce CYP3A4 can significantly affect tadalafil levels in the body, altering its efficacy or risk of side effects.
Excretion
Tadalafil is eliminated mainly through hepatic metabolism, and its metabolites are excreted predominantly in the feces (about two-thirds) and to a lesser extent in the urine (about one-third). The drug has a relatively long elimination half-life of approximately 17.5 hours, which supports its prolonged duration of action of up to 24–36 hours.
PHARMACODYNAMICS
Tadalafil pharmacodynamics involves selective inhibition of phosphodiesterase type 5 (PDE5), leading to increased levels of cyclic guanosine monophosphate (cGMP) in smooth muscle cells. This enhances nitric oxide–mediated vasodilation, resulting in relaxation of vascular smooth muscle, particularly in the corpus cavernosum of the penis. The increased blood flow facilitates erection in response to sexual stimulation. In addition, tadalafil also causes smooth muscle relaxation in the pulmonary vasculature and lower urinary tract, which explains its therapeutic effects in pulmonary arterial hypertension and benign prostatic hyperplasia.
ADMINISTRATION
Tadalafil is administered orally in tablet form and can be taken with or without food, as food does not significantly affect its absorption. It is used either as an on-demand dose taken before sexual activity or as a once-daily low-dose regimen for continuous effect, depending on the indication. The tablet should be swallowed whole with water, and dosing should follow medical guidance to ensure safety and effectiveness.
DOSAGE AND STRENGTH
Tadalafil is available in several oral tablet strengths depending on the indication. For erectile dysfunction, common on-demand doses are 10 mg and 20 mg, taken before sexual activity, while a 2.5 mg or 5 mg once-daily dose may be used for continuous treatment. For benign prostatic hyperplasia, 5 mg once daily is typically prescribed. In pulmonary arterial hypertension (under specialized supervision), higher doses such as 20 mg once daily have been used. Dosage is individualized based on patient response, tolerability, and clinical condition.
DRUG INTERACTIONS
Tadalafil can interact with several medicines, especially those that affect blood pressure or liver metabolism. It should not be used with nitrates (such as nitroglycerin) because this can cause a dangerous drop in blood pressure. Caution is also needed with alpha-blockers, antihypertensives, and strong CYP3A4 inhibitors like ketoconazole, ritonavir, or clarithromycin, as these can increase tadalafil levels and side effects. Alcohol in large amounts may also enhance blood pressure–lowering effects.
FOOD INTERACTIONS
Tadalafil has minimal food interactions and can be taken with or without food without significantly affecting its absorption or effectiveness. However, excessive alcohol consumption should be avoided because it may increase the risk of dizziness, low blood pressure, and reduced erectile response.
CONTRAINDICATIONS
Tadalafil is contraindicated in patients taking any form of nitrates (such as nitroglycerin) or guanylate cyclase stimulators like riociguat because of the risk of severe hypotension. It should also be avoided in individuals with hypersensitivity to tadalafil or any of its components, and used with caution or avoided in patients with significant cardiovascular disease where sexual activity is not advised.
SIDE EFFECTS
Headache
Flushing
Dyspepsia (indigestion)
Back pain
Muscle aches
Nasal congestion
Dizziness
Visual disturbances (rare)
OVER DOSE
Severe headache
Marked hypotension (low blood pressure)
Dizziness or fainting
Palpitations
Flushing
TOXICITY
Tadalafil toxicity occurs mainly due to excessive doses or drug accumulation, leading to strong vasodilation effects such as severe hypotension, dizziness, headache, flushing, palpitations, and sometimes priapism. In severe cases, it may cause cardiovascular instability, especially when combined with interacting drugs like nitrates.
