Prazosin to Terazosin Dose Conversion
There is no established direct dose equivalence between prazosin 5mg four times daily (20mg total daily) and terazosin, but based on pharmacokinetic data showing terazosin has 3-4 times longer half-life than prazosin with similar potency, a reasonable starting conversion would be terazosin 5-10mg once daily, with careful titration based on blood pressure response.
Key Pharmacokinetic Differences
The fundamental challenge in converting between these agents stems from their different pharmacokinetic profiles:
- Terazosin has an elimination half-life approximately 12 hours, which is 2-4 times longer than prazosin 1, 2, 3
- Terazosin is almost completely and consistently absorbed (90-94% bioavailability), whereas prazosin absorption is less predictable 1, 3
- Both drugs have similar pharmacodynamic profiles as selective alpha-1 adrenergic antagonists 1, 4
Clinical Context and Guideline Considerations
Important Limitation
Prazosin has insufficient data to support its use for benign prostatic hyperplasia (BPH) or as a preferred agent for hypertension 5. The AUA guidelines explicitly state that "data are insufficient to support a recommendation for the use of prazosin" for LUTS secondary to BPH 5.
Approved Dosing Ranges
- Prazosin: 2-20mg daily in 2-3 divided doses 5, 6
- Terazosin: 1-20mg once daily 5
- Maximum effective terazosin dose studied is 10mg daily for BPH 5
Practical Conversion Approach
Step 1: Calculate Total Daily Prazosin Dose
Your patient is taking prazosin 5mg QID = 20mg total daily dose
Step 2: Consider Conversion Ratio
Given that:
- Terazosin's longer half-life allows once-daily dosing versus prazosin's multiple daily doses 1, 2
- Comparative studies show terazosin 5.6mg daily achieved similar efficacy to lower doses of comparator alpha-blockers 7
- The pharmacodynamic potency is similar between the two agents 1, 4
Step 3: Recommended Starting Dose
Start terazosin at 5mg once daily at bedtime, then titrate upward by 2-5mg increments every 1-2 weeks based on blood pressure response, up to a maximum of 10-20mg daily 5, 1.
Critical Safety Considerations
Orthostatic Hypotension Risk
- Both prazosin and terazosin are associated with orthostatic hypotension, especially in older adults 5, 6
- First-dose syncope can occur with terazosin but is rare and can be avoided by giving the first dose at bedtime 1
- Monitor blood pressure in both supine and standing positions during dose titration 8, 6
Titration Strategy
- Begin with terazosin 1mg at bedtime for the first dose to minimize first-dose hypotension risk 1, 2
- Increase gradually (1-2mg increments every few days to weeks) until blood pressure is controlled 5, 1
- The mean effective dose in clinical trials was 2.4-5.6mg daily 7
Clinical Caveats
Do not assume a simple 1:1 conversion exists - the longer half-life and more predictable absorption of terazosin means the total daily dose may be lower than the total daily prazosin dose 1, 3. Individual patient response varies significantly, requiring careful monitoring and dose adjustment 5, 1.
If the indication is hypertension, consider switching to a preferred alpha-blocker (doxazosin, tamsulosin, or alfuzosin) rather than terazosin, as these have more robust evidence 5.