Article: Tadalafil

Systematic (IUPAC) name
(6R-trans)-6-(1,3-benzodioxol-5-yl)- 2,3,6,7,12,12a-hexahydro-2-methyl-pyrazino [1', 2':1,6] pyrido[3,4-b]indole-1,4-dione
CAS number 171596-29-5
ATC code G04BE08
PubChem 110635
DrugBank APRD00071
Chemical data
Formula C22H19N3O4
Mol. weight 389.404 g/mol
Pharmacokinetic data
Bioavailability  ?
Protein binding 94%
Metabolism  ?
Half life 17.5 hours
Excretion  ?
Therapeutic considerations
Pregnancy cat.


Legal status
Routes  ?

Tadalafil is a drug used to treat male erectile dysfunction (impotence). In the United States, tadalafil has Food and Drug Administration approval and became available in December, 2003 as the third pill after Viagra and Levitra. Due to its 36-hour effect it is also known as the "Weekend Pill".

It was developed by the biotechnology firm ICOS and marketed worldwide by Eli Lilly and Company under the brand name Cialis.

Tadalafil at work

It works by inhibiting an enzyme known as PDE5. A 20mg dose of Tadalafil is comparable to a 100mg dose of sildenafil (Viagra).

Chemical information

The empirical formula for tadalafil is C22H19N3O4, and its official organic name is (6R,12aR)-6-(1,3-benzodioxol-5-yl)-2,3,6,7,12,12a- hexahydro-2-methyl-pyrazino[1 ,2 :1,6]pyrido[3,4-b]indole-1,4-dione. The molecular weight is 389.41. The tablets are yellow, film-coated, and almond-shaped, and are produced in 5, 10, or 20mg doses.

Side effects

The most common side effects when using tadalafil are headache, indigestion, back pain, muscle aches, flushing, and stuffy or runny nose. These side effects usually go away after a few hours. Patients who get back pain and muscle aches usually get it 12 to 24 hours after taking the drug, and the symptom usually goes away after 48 hours.

In May 2005, the U.S. Food and Drug Administration found that tadalafil (along with other PDE5 inhibitors) could lead to vision impairment in certain patient groups, including diabetics. An investigation is currently ongoing.

Drug interactions - organic nitrates

Since PDE5 inhibitors such as tadalafil may cause transiently low blood pressure (hypotension), organic nitrates should not be taken for at least 48 hours after taking the last dose of tadalafil. Using organic nitrates within this timeframe may increase the risk of life-threatening hypotension.

Since people who have taken tadalafil within the past 48 hours cannot take organic nitrates to relieve angina, these patients should seek immediate medical attention if they experience anginal chest pain [1]. In the event of a medical emergency, paramedics and medical personnel should be notified of any recent doses of tadalafil.


In the United States, Eli Lilly has a multiyear agreement to promote tadalafil (Cialis) with professional golf's PGA TOUR.


  • Some individuals with the surname of "Cialis" objected to the naming of Lilly's drug, but the pharmaceutical giant insists that the drug's name has nothing to do with the surname. [2]
  • The word "specialist" is frequently caught by primitive spam email blockers because it contains the word "cialis."