Silodosin shows high affinity and selectivity for α1A adrenergic receptors found in the prostate which ensures that it works quickly and effectively to relieve the symptoms of BPH. Silodosin's low affinity for α1B receptors in the blood vessels is thought to be reflected in its low incidence of orthostatic and vasodilatory side effects.
Tamsulosin is relatively selective for α1a-adrenergic receptors, which are mainly present in the prostate. Hence, it may have a more selective action in BPH with minimal effects on blood pressure.
By reducing α1-adrenergic activity of the blood vessels, these drugs may cause hypotension (low blood pressure) and interrupt the baroreflex response. In doing so, they may cause dizziness, lightheadedness, or fainting when rising from a lying or sitting posture (known as orthostatic hypotension or postural hypotension). For this reason, it is generally recommended that alpha blockers should be taken at bedtime. Additionally, the risk of first dose phenomenon may be reduced by starting at a low dose and titrating upwards as needed.
As discussed above, tamsulosin may have less risk for low blood pressure and orthostatic hypotension due to its selectivity for α1a-adrenergic receptors. On the other hand, the drug (a) elevates risk for floppy iris syndrome, and (b) might show adverse drug reactions (ADRs) characteristic of the sulfa related drugs.
^Yilmaz E, Batislam E, Basar MM, Tuglu D, Ferhat M, Basar H (June 2005). "The comparison and efficacy of 3 different alpha1-adrenergic blockers for distal ureteral stones". J. Urol.173 (6): 2010–2. doi:10.1097/01.ju.0000158453.60029.0a. PMID15879806.
^Nomiya M, Yamaguchi O. A quantitative analysis of mRNA expression of alpha 1 and beta-adrenoceptor subtypes and their functional roles in human normal and obstructed bladders. J Urol. 2003;170(2 Pt 1):649-653.