Equivalent Dosing: Tamsulosin 0.4 mg to Doxazosin
There is no established equivalent dose conversion between tamsulosin 0.4 mg daily and doxazosin because these alpha-blockers have fundamentally different receptor selectivity profiles, pharmacokinetics, and clinical effects that make direct dose equivalency clinically meaningless.
Why Direct Dose Conversion Is Not Appropriate
Tamsulosin is a highly selective alpha-1A and alpha-1D receptor antagonist with preferential action on prostatic smooth muscle, while doxazosin is a non-selective alpha-1 blocker affecting all alpha-1 receptor subtypes equally throughout the vascular system 1, 2.
The pharmacokinetic profiles differ substantially: tamsulosin has a modified-release formulation designed for once-daily dosing with minimal blood pressure effects, whereas doxazosin has maximal hypotensive effects 2-4 hours post-dose with significant cardiovascular activity 1, 3.
Tamsulosin has significantly lower probability of orthostatic hypotension compared to doxazosin, making them non-interchangeable from a safety perspective 4, 1.
Clinical Approach If Switching Is Necessary
For BPH Treatment
If switching from tamsulosin 0.4 mg to doxazosin for BPH, start doxazosin at 1 mg once daily at bedtime and titrate upward by 1-2 mg every 1-2 weeks as tolerated, with typical effective doses ranging from 4-8 mg daily 3, 5.
Doxazosin requires initial low-dose titration to minimize first-dose orthostatic hypotension, unlike tamsulosin which does not require dose titration 1, 3.
Monitor standing blood pressure at 1 and 3 minutes after standing during the titration period, especially in elderly patients 4.
Critical Safety Considerations
Doxazosin is strongly associated with orthostatic hypotension, particularly in older adults, and should be avoided in patients with baseline standing systolic BP <110 mmHg 4.
Elderly patients and those on multiple medications face highest risk for orthostatic hypotension and falls when using doxazosin 6, 4.
Do not assume equivalent efficacy at any dose ratio—the therapeutic response must be assessed independently based on symptom improvement and urodynamic parameters 1, 5.