The Core Mechanism: How Tadalafil Works
At the heart of tadalafil's effects is its role as a selective phosphodiesterase type 5 (PDE5) inhibitor. This enzyme normally breaks down cyclic guanosine monophosphate (cGMP), a molecule that relaxes smooth muscles in blood vessel walls, increasing blood flow. Tadalafil prevents the breakdown of cGMP, enhancing its effects. This leads to vasodilation, or the widening of blood vessels. Since PDE5 is found throughout the body, tadalafil has widespread effects, not just in targeted areas.
Effects of Tadalafil on Sexual Function
Tadalafil is widely known for treating erectile dysfunction (ED). When sexually stimulated, the inhibition of PDE5 by tadalafil increases cGMP levels in the penis. This causes the smooth muscles in the penile arteries and corpus cavernosum to relax, boosting blood flow. This increased blood flow, along with sexual stimulation, facilitates a firmer and more sustained erection. Tadalafil's effects can last up to 36 hours, allowing for greater spontaneity compared to some other ED medications. Remember that sexual arousal is necessary for tadalafil to work.
Effects on Benign Prostatic Hyperplasia (BPH)
Tadalafil is also approved for treating the symptoms of an enlarged prostate, known as BPH. It can be used to manage both ED and BPH in men experiencing both conditions. The drug works by relaxing the smooth muscles in the bladder, prostate, and urethra. This helps to relieve urinary symptoms associated with BPH, such as frequent or weak urination and incomplete bladder emptying.
Effects on Pulmonary Arterial Hypertension (PAH)
A specific formulation of tadalafil (marketed as Adcirca) is used for treating pulmonary arterial hypertension (PAH). In this case, tadalafil relaxes the blood vessels in the lungs, which reduces blood pressure in the pulmonary arteries and makes it easier for the heart to pump blood. This can improve exercise capacity and overall well-being in patients with PAH.
Systemic Effects and Side Effects
Tadalafil's vasodilating effects occur throughout the body, leading to potential systemic effects and side effects. Most side effects are typically mild and improve with continued use.
Common Side Effects (usually mild to moderate)
- Headache
- Indigestion
- Back pain and muscle aches
- Flushing
- Nasal congestion
Rare and Serious Side Effects
- Priapism (prolonged erection)
- Sudden vision loss
- Sudden hearing loss
- Significant drop in blood pressure, especially when taken with nitrates
Tadalafil vs. Other PDE5 Inhibitors
Tadalafil differs from other PDE5 inhibitors like sildenafil (Viagra) primarily in its duration of action and enzyme selectivity. Refer to the table below for a comparison of key features.
| Feature | Tadalafil | Sildenafil | Vardenafil | Avanafil (Stendra) |
|---|---|---|---|---|
| Onset of Action | Can be as fast as 30 min, typically ~2 hours. | ~60 minutes. | ~15-60 minutes. | ~15-30 minutes. |
| Duration of Action | Up to 36 hours (as-needed); continuous (daily). | ~4-6 hours. | ~4-8 hours. | ~4-6 hours. |
| Effect of Food | Negligible effect on absorption. | Decreased absorption with fatty meal. | Decreased absorption with fatty meal. | Negligible effect on absorption. |
| Common Side Effects | Headache, back pain, muscle aches, dyspepsia. | Headache, flushing, nasal congestion, vision changes. | Headache, flushing, dyspepsia, nasal congestion. | Headache, flushing, nasal congestion. |
| Selectivity | High selectivity for PDE5, less inhibitory for PDE6 (which affects vision). | Also inhibits PDE6, which may cause vision changes. | Also inhibits PDE6, though less frequently causes vision changes than sildenafil. | Highly selective for PDE5. |
The longer half-life of tadalafil contributes to its extended effects compared to sildenafil.
Conclusion
Taking tadalafil results in the inhibition of the PDE5 enzyme, increasing cGMP and relaxing smooth muscles, thereby enhancing blood flow. This mechanism is effective for treating erectile dysfunction, BPH symptoms, and pulmonary arterial hypertension. While generally safe, potential side effects can occur, ranging from common mild effects like headache to rare serious issues such as vision or hearing loss. Tadalafil's long duration of action offers flexibility in use. Consult a healthcare provider to determine if tadalafil is appropriate for you.
Further reading
- U.S. National Library of Medicine: Tadalafil in the treatment of erectile dysfunction [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2643112/]
