Hydralazine
Contraindicated
Textbook
Potentially excessive vasodilatation and compensatory effects.
Avoid combination.
Source: KDT 7e · p571
Drug reference
Prazosin
Selective alpha-1 adrenoceptor antagonist · Antihypertensive, Benign prostatic hyperplasia treatment
START
0.5–1 mg at bedtime (first-dose effect); PTSD 1 mg nocte
TYPICAL MAX
20 mg/day (HTN); titrate slowly
STOP IF
Recurrent syncope or intolerable orthostasis
WATCH
Postural BP, first-dose hypotension, IFIS if cataract surgery planned
CDSCO approvedSchedule HATC C02CA01
KDIGO 2024 + manufacturer label
- ONSET
- 2h · BP onset
- PEAK
- 2.5h · Cmax
- t½
- 2.5h · plasma t½
- DURATION
- 12h · effect duration
EXCRETION
Hepatic metabolism; mainly biliary, <10% renal
route + CYP
INTERACTIONS
5 major
incl. contraindicated
PREGNANCY
Use only if clearly needed — limited human data; not first-line
FDA category + note
Top interactionssee all 10 →
- HydralazineContraindicatedTextbookKDT 7e · p571
- Pde5 InhibitorsSevereDatabaseKimi deep-research + Cla
- SilodosinSevereDatabaseDDInter
- Sodium OxybateSevereDatabaseDDInter
Available in India
58 branded formulations. Look up specific brands in the Drugs workspace.
Mechanism
Competitively blocks postsynaptic alpha-1 adrenoceptors, relaxing arteriolar and venous smooth muscle (reduced peripheral resistance) and prostatic/bladder-neck smooth muscle. Central alpha-1 blockade underlies its efficacy in PTSD-related nightmares.
Indications
HypertensionBenign prostatic hyperplasiaPTSD-associated nightmares (off-label, well established)Raynaud phenomenon (adjunct)
Dosing
- Adult
- Hypertension: 0.5–1 mg PO at bedtime (first-dose), titrate to 2–20 mg/day in 2–3 divided doses. PTSD nightmares: 1 mg at night, titrate to 3–15 mg.
- Pediatric
- 0.05–0.1 mg/kg/day divided (specialist).
- Renal adjustment
- Initiate at low dose; no formal mg adjustment but titrate cautiously.
- Hepatic adjustment
- Use with caution; lower starting dose (extensive hepatic metabolism).
- Geriatric
- Start 0.5 mg at night; heightened first-dose hypotension/fall risk.
- Max dose
- 20 mg/day (hypertension); some sources up to 40 mg/day specialist
Pharmacokinetics
Onset
~2 h (BP)
Peak effect
2–3 h
Duration
~10–24 h
Half-life
2–3 h
Bioavailability
~50–70%
Protein binding
~95%
Metabolism
Extensive hepatic (demethylation/conjugation)
Excretion
Mainly biliary/faecal; <10% renal
Contraindications
- Hypersensitivity to prazosin or quinazolines
- Caution: history of micturition/postural syncope
Side effects
Common
First-dose orthostatic hypotension/syncopeDizziness, headacheDrowsiness, fatigueNasal congestionPalpitations
Serious
- Severe first-dose syncope
- Priapism (rare)
- Intraoperative floppy iris syndrome
Pregnancy & lactation
Pregnancy
Use only if clearly needed — limited human data; not first-line
Lactation
Limited data; small amounts in milk — caution
Drug interactions
Pde5 Inhibitors
Severe
Database
Additive vasodilation → symptomatic hypotension
Stable alpha-blocker dose, low PDE5i dose, separate timing
Source: Kimi deep-research + Cla
Silodosin
Severe
Database
Increased risk of orthostatic hypotension, dizziness, and syncope due to additive alpha-adrenergic blockade.
Concomitant use with other alpha-blockers is contraindicated due to the risk of severe hypotension. If a change in alpha-blocker is needed, ensure a washout period.
Source: DDInter
Sodium Oxybate
Severe
Database
Clinical effect not specified
Source: DDInter
Tizanidine
Severe
Database
Clinical effect not specified
Source: DDInter
Atenolol
Moderate
Database
Increased antihypertensive effect.
This combination is often used to achieve better blood pressure control and manage reflex tachycardia from alpha-1 blockers.
Source: DDInter
Clonidine
Moderate
Database
Reduced antihypertensive efficacy of both
Avoid combination.
Source: DDInter
Nadolol
Moderate
Database
Increased antihypertensive effect.
This combination is often used to achieve better blood pressure control and manage reflex tachycardia from alpha-1 blockers.
Source: DDInter
Pindolol
Moderate
Database
Increased antihypertensive effect.
This combination is often used to achieve better blood pressure control and manage reflex tachycardia from alpha-1 blockers.
Source: DDInter
Propranolol
Moderate
Database
Increased antihypertensive effect.
This combination is often used to achieve better blood pressure control and manage reflex tachycardia from alpha-1 blockers.
Source: DDInter
2 additional low-confidence interactions hidden — those rows lack a documented mechanism or management plan in our sources.
Related guidelines
Ask House about Prazosin
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Sources: KD Tripathi 7e, Goodman & Gilman 14e, BNF·Verified: 2026-05-19 · House clinical team·Cockpit curated: 2026-05-19