Tadalafil is a selective, reversible inhibitor of cyclic guanosine monophosphate (cGMP)-specific phosphodiesterase type 5 (PDE5). The mean half-life of the drug is 17.5 hours in people who are healthy and steady-state plasma concentrations are attained within 5 days of once-daily dosing. PDE5 is an enzyme found in the corpus cavernosum smooth muscle, as well as in the smooth muscle of the prostate, the bladder and their vascular supply (Cialis summary of product characteristics).
The exact mechanism by which tadalafil affects symptoms associated with benign prostatic hyperplasia is not known. The summary of product characteristics for tadalafil describes how inhibition of PDE5 causes relaxation in the vascular supply of the prostate and bladder that increases blood perfusion to these areas, which may be the mechanism by which symptoms of benign prostatic hyperplasia are reduced. These vascular effects may be complemented by inhibition of bladder afferent nerve activity and smooth muscle relaxation of the prostate and bladder.
In October 2012, tadalafil (Cialis, 5 mg tablet only, Eli Lilly and Company Ltd) received a European Union marketing authorisation for treating the signs and symptoms of benign prostatic hyperplasia in adult men (Eli Lilly UK: personal communication January 2013).
Previously, in November 2002, tadalafil (Cialis, 2.5 mg, 5 mg, 10 mg and 20 mg tablets, Eli Lilly and Company Ltd) received a European Union marketing authorisation for treating erectile dysfunction in adult men.
The prescribing of tadalafil for erectile dysfunction in England is subject to statutory prescribing restrictions through Schedule 2 of the NHS (general medical services contracts) (prescription of drugs etc.) regulations 2004 and the NHS (Pharmaceutical and Local Pharmaceutical Services) Regulations 2013. The regulations specify in which circumstances prescriptions for certain erectile dysfunction treatments can be reimbursed by the NHS in primary care. Details of the clinical conditions that are covered by the regulations scheme are reproduced in Part XVIIIB of the NHS electronic drug tariff - 'Drugs, medicines and other substances that may be ordered only in certain circumstances'.
These prescribing restrictions do not apply to tadalafil when it is prescribed in primary care on the NHS for benign prostatic hyperplasia (Department of Health: personal communication October 2013).
The summary of product characteristics for tadalafil advises that the dose of tadalafil for benign prostatic hyperplasia in adult men is 5 mg taken approximately at the same time every day with or without food. This is also the dose for men with both benign prostatic hyperplasia and erectile dysfunction. The efficacy of tadalafil 2.5 mg for treating benign prostatic hyperplasia has not been demonstrated.
When tadalafil is used for treating erectile dysfunction, it can be taken on an as needed basis (10 mg and 20 mg strengths), or as a regular once-daily dose (2.5 mg and 5 mg strengths). To treat benign prostatic hyperplasia, tadalafil should be taken regularly once a day.
A 28-tablet pack of tadalafil 5 mg (Cialis, Eli Lilly and Company Ltd) costs £54.99, representing an annual cost of £716.83 (taken from the NHS drug tariff February 2013).