Prazosin
Mechanism :
Prazosin causes a decrease in total peripheral resistance. Studies suggest that the vasodilator effect of prazosin is related to selective blockade of post-synaptic alpha1-adrenoceptors. In patients with hypertension there is little change in cardiac output. In addition, clinical pharmacology studies have shown that prazosin antagonizes the vasopressor effect of i.v. phenylephrine, an alpha1-agonist.
Indication :
Contraindications :
Hypersensitivity to prazosin or related quinazolines. Not recommended for congestive heart failure due to mechanical obstruction. When giving first dose monitor for excessive hypotension or give at bedtime.
Dosing :
Initially 0.05-0.1 mg/kg/day orally in divided 8 hourly doses; titrate to 0.5 mg/kg/day.
Maximum dose: 20 mg/day.
Adverse Effect :
Vomiting, diarrhea, constipation, abdominal discomfort and/or pain, syncope, orthostatic hypotension, edema, dyspnea, tachycardia, faintness, nervousness, vertigo, depression, paresthesia, hallucinations, rash, pruritus, alopecia, lichen planus, urinary frequency, incontinence, impotence, priapism, blurred vision, reddened sclera, epistaxis, tinnitus, nasal congestion, liver function abnormalities, pancreatitis, decreased hematocrit/hemoglobin, diaphoresis, fever, arthralgia, positive ANA titre.
Interaction :
Prazosin has been administered without any adverse drug interaction in limited clinical experience to date with the following: cardiac glycosides - digitalis and digoxin; hypoglycemics - insulin, chlorpropamide, tolazamide and tolbutamide; tranquilizers and sedatives - chlordiazepoxide, diazepam and phenobarbital; antigout - allopurinol, colchicine and probenecid; antiarrhythmics - procainamide, propranolol and quinidine; and analgesics, antipyretics and anti-inflammatories - propoxyphene, ASA, indomethacin and phenylbutazone.
Diuretic or other Antihypertensive Agent: Addition of a diuretic or other antihypertensive agent to prazosin has been shown to cause an additive hypotensive effect.
Renal Dose :
Dose in Renal Impairment GFR (mL/min)
| 20-50 | Dose as in normal renal function |
| 10-20 | Dose as in normal renal function |
| <10 | Dose as in normal renal function |
Dose in Patients undergoing Renal Replacement Therapies
| CAPD | Not dialysed. Dose as in normal renal function |
| HD | Not dialysed. Dose as in normal renal function |
| HDF/High flux | Unknown dialysability. Dose as in normal renal function |
| CAV/VVHD | Not dialysed. Dose as in normal renal function |
Hepatic Dose :
Drug is extensively metabolized in the liver, therefore initiation with lower dose is reasonable. Titration of dose should be done while monitoring clinical response.