Drug information of Prazosin

Mechanism of effect

The exact mechanism of the hypotensive action of Prazosin is unknown. Prazosin causes a decrease in total peripheral resistance and was originally thought to have a direct relaxant action on vascular smooth muscle. Alpha-1 blocker inhibits postsynaptic alpha-adrenergic receptors, causing arterial and venous dilation and a subsequent decrease in blood pressure

Pharmacodynamic

Prazosin is a sympatholytic drug used to treat high blood pressure , anxiety, and posttraumatic stress disorder (PTSD) . It is anα1-blocker which acts as an inverse agonist at alpha-1 adrenergic receptors. These receptors are found on vascular smooth muscle, where they are responsible for the vasoconstrictive action of norepinephrine. They are also found throughout the central nervous system .
Prazosin is orally active and has a minimal effect on cardiac function due to its alpha-1 receptor selectivity. However, when prazosin is started, heart rate and contractility go up in order to maintain the pre-treatment blood pressures because the body has reached homeostasis at its abnormally high blood pressure. The blood pressure lowering effect becomes apparent when prazosin is taken for longer periods of time. The heart rate and contractility go back down over time and blood pressure decreases.
The antihypertensive characteristics of prazosin make it a second-line choice for the treatment of high blood pressure.
Prazosin is also useful in treating urinary hesitancy associated with prostatic hyperplasia, blocking alpha-1 receptors, which control constriction of both the prostate and urethra. Although not a first line choice for either hypertension or prostatic hyperplasia, it is a choice for patients who present with both problems concomitantly.

Pharmacokinetics

Absorption :
• Bioavailability: 43-82%
• Onset: ~2 hr
• Biological half-life : 2–3 hours
• Peak plasma time: 2-3 hr
Distribution:
Protein bound: 97%
Metabolism:
Hepatic metabolization (extensive)
Elimination:
Half-life elimination: 2-3 hr
Excretion: Urine (6-10%); rest in feces (via bile)

Drug indications

(hypertension (high blood pressure , Benign Prostatic Hyperplasia

Dosage

Tablet : 1mg 2mg 5mg
• Usual Adult Dose for Hypertension
Initial dose: 1 mg orally 2 or 3 times a day
Maintenance dose: 1 to 20 mg orally per day in divided doses
• Comments:
- Titrate slowly as determined by blood pressure response.
- Therapeutic dosages usually range from 6 to 15 mg per day in divided doses.
- Total daily doses greater than 20 mg usually do not increase efficacy, but some patients may benefit from daily doses up to 40 mg per day in divided doses.
• Renal Dose Adjustments: Data not available
• Liver Dose Adjustments: Data not available
• Dose Adjustments:
• Adding a diuretic or another antihypertensive agent:
-Reduce the dose of this drug to 1 or 2 mg three times a day and then retitrate.
• Coadministration with phosphodiesterase-5 (PDE-5) inhibitors:
-Initiate the PDE-5 inhibitor at the lowest dose.

Drug contraindications

Hypersensitivity to this drug

Side effects

nausea , Headache , dry mouth , abdominal pain , vomiting , Blurred vision , irregular or fast heart rate , vertigo , Depression , Diarrhea , asthenia , edema , fever , impotence , liver failure , itching , Urinary frequency , difficulty urinating , tiredness , swelling of the face , swelling of the tongue , tightness in the chest , Fast or irregular heartbeat , Rash , Abdominal discomfort , stuffy nose , Syncope , severe or persistent dizziness

Interactions

Estrogen , Indomethacin , Chlorpromazine , Pindolol , Bendroflumethiazide , Treprostinil , cyclopenthiazide

Alerts

some medical conditions may interact with prazosin. Tell your doctor or pharmacist if you have any medical conditions, especially if any of the following apply to you:
• if you are pregnant, planning to become pregnant, or are breast-feeding
• if you are taking any prescription or nonprescription medicine, herbal preparation, or dietary supplement
• if you have allergies to medicines, foods, or other substances
• if you have chest pain, heart disease, narcolepsy, or kidney problems • if you will be having eye surgery Once you have started Pegasys treatment, talk to your doctor, nurse or pharmacist:
• if you develop symptoms associated with depression (e.g. feelings of sadness, dejection, etc.)
• if you notice a change in your vision.
• if you develop symptoms associated with a cold or other respiratory infection (such as cough, fever or any difficulty in breathing).
• if you think you are getting an infection (such as pneumonia) as when receiving Pegasys you may temporarily have a greater risk of getting an infection.
• if you develop any signs of bleeding or unusual bruising, check with your doctor immediately.
• if you develop signs of a severe allergic reaction (such as difficulty in breathing, wheezing or hives) while on this medication, seek medical help immediately.

Points of recommendation

Storage requirements: Protect fromlight.
Patient advice:
- Advise patients how to avoid symptoms associated with lowering of blood pressure (e.g., dizziness, lightheadedness) and what to do if they occur. Also, encourage patients to avoid situations where injury could result if these symptoms occur.
- Inform male patients about the seriousness of priapism and instruct them to seek immediate medical attention if it occurs.
- Advise patients to report current or previous use of this drug to their ophthalmologist.

Pregnancy level

C

Related drugs

Tamsulosin , Terazosin , Doxazosin , Alfuzosin