Prazosin tablets (all strengths)
- Drugs List
- Therapeutic Indications
- Precautions and Warnings
- Pregnancy and Lactation
- Side Effects
Oral formulations of prazosin.
Benign prostatic hyperplasia
Congestive heart failure
For the treatment of all grades of hypertension
For the treatment of congestive heart failure in patients who are resistant or refractory to conventional therapy with diuretics and/or cardiac glycosides.
For the symptomatic treatment of Raynaud's phenomenon and Raynaud's disease
As an adjunct in the symptomatic treatment of urinary obstruction caused by benign prostatic hyperplasia.
The usual dosage range is 500micrograms to 20mg daily.
Treatment should be initiated at the lowest dose, 500micrograms two to three times a day for 3 to 7 days. The starting dose should be administered in the evening. This dose may then be increased to 1mg two to three times a day for a further 3 to 7 days.
Thereafter, the dose should be increased gradually as determined by clinical response.
Most patients can be maintained on up to 15mg daily in divided doses. The maximum daily dose is 20mg in divided doses.
Patients Receiving Other Antihypertensive Therapy but with Inadequate Control
The dosage of the other drug should be reduced to a maintenance level and prazosin initiated at 500micrograms in the evening. This dose may then be titrated to 500micrograms two to three times a day, with subsequent dose increases made according to clinical response.
There is evidence that some patients concurrently taking other antihypertensive agents may respond to the first dose of treatment in an abrupt and exaggerated manner. Side effects may be avoided by initiating treatment at the lowest dose with small increases in dosage during the first few weeks.
Congestive Cardiac Failure
The recommended starting dose is 500micrograms two to four times a day, increased to 4mg in divided doses. Dosage should be adjusted according to the patient's clinical response. Dosage may be adjusted every 2 to 3 days under close medical supervision. In severely ill, decompensated patients, rapid dosage adjustment over 1 to 2 days with close haemodynamic monitoring may be necessary.
Usual maintenance dose: 4mg to 20mg daily in divided doses.
Raynaud's Phenomenon and Raynaud's Disease
The recommended starting dose is 500micrograms twice daily for a period of 3 to 7 days, and should then be adjusted according to clinical response. The usual maintenance dose is 1mg to 2mg twice daily.
Benign Prostatic Hyperplasia
The recommended dose is 500micrograms twice daily for 3 to 7 days, with the initial dose administered in the evening. The dose should then be titrated according to clinical response. The usual maintenance dose is 2mg twice daily. This dose should not be exceeded unless the patient also requires antihypertensive therapy.
(See Dosage; Adult).
Initiate therapy at the lowest dose, as the elderly are more susceptible to hypotension.
Children aged 12 years and over
(See Dosage; Adult).
Children aged 1 month to 12 years (unlicensed)
Hypertension:10micrograms/kg to 15micrograms/kg two to four times a day (initial dose at bedtime), increased gradually to maximum 500micrograms/kg daily in divided doses (not exceeding 20mg daily).
Congestive Cardiac Failure: 5micrograms/kg twice daily (initial dose at bedtime) increased gradually to maximum 100micrograms/kg daily in divided doses.
Patients with Renal Impairment
Moderate to severe renal impairment:
Therapy should be initiated at 500micrograms daily and increased cautiously.
Patients with Hepatic Impairment
Therapy should be initiated at 500micrograms daily and increased cautiously.
Congestive cardiac failure secondary to mechanical obstruction
History of micturition syncope - if treating benign prostatic hyperplasia
Precautions and Warnings
Children aged 1 month to 12 years
Moderate renal impairment
Advise ability to drive/operate machinery may be affected by side effects
First dose should be taken at bed-time
Monitor blood pressure during titration and early maintenance treatment
Intraoperative Floppy Iris Syndrome has been reported in cataract surgery
Use low starting dose to avoid postural effects
A low starting dose is recommended in order to reduce the risk of orthostatic hypotension or loss of consciousness. Starting dose should be taken on retiring to bed, due to risk of collapse.
Prazosin may augment the efficacy of antihypertensive therapy; consequently close observation is recommended especially for patients taking concurrent medications known to lower blood pressure.
Clinical efficiency of prazosin in congestive cardiac failure may diminish after several months treatment in some patients, however a causal relationship to prazosin therapy has not been established. If there is evidence of weigh gain or peripheral oedema careful adjustment of diuretic dosage according to the patient's clinical condition may be required.
In patients without evidence of fluid retention, when clinical improvement has diminished, an increase in dosage of prazosin will usually restore clinical efficiency.
Patients with benign prostatic hyperplasia receiving antihypertensive therapy should be administered prazosin only under the supervision of the practitioner responsible for treating the patient's hypertension.
Isolated reports of Intraoperative Floppy Iris Syndrome (IFIS) have been associated with other alpha 1 blockers and the possibility of a class effect cannot be excluded. As IFIS may lead to increased procedural complications during cataract operations, current or past use of alpha 1 blockers should be notified to the ophthalmic surgeon.
Pregnancy and Lactation
Use prazosin with caution in pregnancy.
There is insufficient study data to conclude safety of prazosin during pregnancy.
Safety has not been established, however no adverse effects have been reported when prazosin has been used alone or in combination during a limited number of pregnancies.
No evidence of teratogenic effects have been observed in reproduction studies in rats, rabbits and monkeys at doses of more than 225, 225 and 12 times the maximum human recommended dose (MHRD), respectively. It is not known if prazosin crosses the placental barrier, however the molecular weight (384) is low enough for this to be expected (Briggs, 2015). Prazosin should only be used in pregnancy when the benefits outweigh the potential risk. Schaefer (2015) concludes that prazosin should only be considered for 2nd or 3rd trimester use if the antihypertensive drug of choice has failed.
The use of all medication in pregnancy should be avoided whenever possible; particularly in the first trimester. Non-drug treatments should also be considered. When essential, a medication with the best safety record over time should be chosen, employing the lowest effective dose for the shortest possible time. Polypharmacy should be avoided. Teratogens taken in the pre-embryonic period, often quoted as lasting until 14 to 17 days post-conception, are believed to have an all-or-nothing effect. Where drugs have a short half-life, and when the date of conception is certain, this may allow women to be reassured where drug exposure has occurred within this time frame. Further advice may be available from the UK National Teratology Information Service (NTIS) and through ToxBase, available via password on the internet ( www.toxbase.org ) or if this is unavailable at the backup site ( www.toxbasebackup.org ).
Use prazosin with caution in breastfeeding.
Prazosin has been to shown to be excreted in small amounts in breast milk, with a low molecular weight of 384. The effects on the nursing infant from exposure to the drug from breast milk are unknown. Antihypertensives may reduce breast milk production. An alternative antihypertensive drug may be preferred, especially while nursing a newborn or premature infant.
Neonates, infants born prematurely, those with low birth weight, those with an unstable gastrointestinal function or who have serious illnesses may require special consideration. For any infant, if a drug is prescribed to the nursing mother, it should be at the lowest practical dose and for the shortest time. When drug administration is unavoidable and breastfeeding is to continue, minimisation of exposure of the infant to the drug may sometimes be achieved by timing the maternal doses to just after a feeding episode. Infants exposed to drugs via breast milk should be monitored for unusual signs or symptoms. Interactions between the drug received by the infant from the mother's milk and medication prescribed for the infant should also be considered, for example, when the drug given to the infant may prevent metabolism of the drug received via breast milk.
Specialist advice is available from the UK Drugs in Lactation Advisory Service at https://www.midlandsmedicines.nhs.uk/content.asp?section=6&subsection=17&pageIdx=1
Exacerbation of narcolepsy
Liver function disturbances
Positive ANA titre
It is strongly recommended that the UK National Poisons Information Service be consulted on cases of suspected or actual overdose where there is doubt over the degree of risk or about appropriate management.
The following number will direct the caller to the relevant local centre (0844) 892 0111
Information may be obtained if you have access to ToxBase the primary clinical toxicology database of the National Poisons Information Service. This is available via password on the internet ( www.toxbase.org ) or if this is unavailable at the backup site ( www.toxbasebackup.org ).
Last Full Review Date: October 2016
Drugs During Pregnancy and Lactation: Treatment Options and Risk Assessment, 3rd edition (2015) ed. Schaefer, C., Peters, P. and Miller, R. Elsevier, London.
Drugs in Pregnancy and Lactation: A Reference Guide to Fetal and Neonatal Risk, 10th edition (2015) ed. Briggs, G., Freeman, R. Wolters Kluwer Health, Philadelphia.
Medications and Mothers' Milk, Sixteenth Edition (2014) Hale, T and Rowe, H, Hale Publishing, Plano, Texas.
Summary of Product Characteristics: Hypovase 0.5mg tablets. Pfizer Limited. Revised May 2017.
Summary of Product Characteristics: Hypovase 1mg tablets. Pfizer Limited. Revised May 2017.
NICE Evidence Services Available at: www.nice.org.uk Last accessed: 12 September 2017
US National Library of Medicine. Toxicology Data Network. Drugs and Lactation Database (LactMed).
Available at: https://toxnet.nlm.nih.gov/cgi-bin/sis/htmlgen?LACT
Prazosin. Last revised: 10th March 2015
Last accessed: 6th October 2016
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