Medical treatments for BPH have been shown in various trials to have benefit in reducing the clinical symptoms associated with prostate enlargement. Two classes of drugs are currently in wide use; alpha(1)-adrenergic-receptor antagonists (alpha(1)ARAs) also called; Alpha-adrenergic blockers (or simply; “alpha-blockers”) (alfuzosin, doxazosin, tamsulosin, and terazosin) and 5alpha-reductase inhibitors (5ARIs) (“anti-androgen”) (finasteride and dutasteride).
The first class of drugs; alpha-blockers work through the relaxation of smooth muscle which has the effect of reducing the restrictive impact that an enlarged prostate has on the flow of urine through the urethra. The second class of drugs; 5ARI’s, work by reducing the size of the prostate. Where the effects of alpha-blockers can be seen within weeks, anti-androgen drugs take longer before the any beneficial effects are noticed but arguably get to the root cause of the problem in a more direct way and thus may have a more long-term advantage.
The use of alpha-blockers has been show to reduce total symptom scores by 10% to 20% compared with placebo.
The differing mechanisms of action between the two classes of drugs create the opportunity for possible synergy in combined use. It has been generally demonstrated that for those men with moderate to severe symptoms (prostate volumes 55ml or larger) who choose long-term therapy by pharmaceuticals, a combination of both drugs may be beneficial. The alpha-blockers provide near-term relief and together, the combined effect is better than either drug used independently. Generally the use of alpha-blockers can be discontinued after a few months.
Within each class of drugs there are differences in the side-effect profiles. Of the alpha-blockers; doxazosin and terazosin are associated with dizziness, fatigue and hypotension. Alfuzosin is associated with hypotension as well, although less than doxazosina and terazosin. Tamsulosin provides fast and lasting results with minimal risk of hypotension but tends to cause ejaculatory dysfunction, with 90% having a decrease in ejaculatory volume and 35% of patients having no ejaculate. Anti-androgen drugs tend to cause erectile and ejaculatory dysfunction as well as reduction in libido which happens in 1% to 8% of patients.2 
The benefits that have been identified with combination therapy include a reduction in the need for surgery by half and a halting of symptom progression in two-thirds of men.
 GM Clifford et al. Eur Urol. 2000 Jul;38(1):2-19
 Bar-Yosef Y et al, Harefuah. 2008 Jun;147(6):514-9, 574.
 F Guiliano, BJU Int. 2006 Apr;97 Suppl 2:34-8; discussion 44-5
 YT Logan et al. Am J Geriatr Pharmacother. 2005 Jun;3(2):103-14