What is it used for?

• High blood pressure (hypertension).
• Heart failure.
• Condition called Raynaud's disease or phenomenon, in which the arteries supplying blood to the hands and feet go into spasm, causing pale, cold and painful fingers and toes.
• Enlarged prostate gland (benign prostatic hyperplasia).

How does it work?

• Alpha receptors are found on the muscle in the walls of blood vessels. When prazosin blocks these receptors it causes the muscle in the blood vessel to relax and the blood vessel to widen. This lets the blood pass more easily through the blood vessels and hence reduces the pressure in the blood vessels. Prazosin can therefore be used to treat high blood pressure.
• Widening the blood vessels also decreases the effort required by the heart to pump blood around the body, because there is less resistance for the heart to pump against. Prazosin can therefore also help treat heart failure, where the pumping action of the heart has become less effective.
• Prazosin is also used to treat Raynaud's disease and Raynaud's phenomenon, which are conditions where the arteries supplying blood to the hands and feet go into spasm and narrow. This restricts the blood supply to the fingers and toes and causes them to go pale, cold and painful. Prazosin widens the blood vessels and so increases the blood flow to the hands and feet, thus relieving these symptoms.
• Alpha receptors are also found on the muscle in the prostate gland. This gland is found only in men and lies at the top of the tube connecting the bladder to the outside (urethra). The prostate gland often enlarges with advancing age (benign prostatic hyperplasia), pressing on the urethra and obstructing the flow of urine from the bladder. This can cause various urinary symptoms such as difficulty passing urine. By blocking the alpha receptors in the prostate, prazosin causes the muscle to relax. This allows urine to flow freely past the prostate gland and relieves the urinary symptoms of this condition.

Important Information

• The first dose of this medicine may cause dizziness, light-headedness or fainting. For this reason you should take the first dose just before retiring to bed at night. Take care when moving from lying down to sitting or standing, particularly if you wake up in the night, until you know how this medicine affects you. If you feel dizzy or light-headed at any time during treatment, sit or lie down until the symptoms pass.

Use with caution in

• Elderly people.
• Decreased kidney function.
• Decreased liver function.
• People taking blood pressure lowering medication.

Not to be used in

• Allergy to related medicines (quinazolines), eg doxazosin, terazosin.
• This medicine is not recommended for people who have ever fainted after passing urine (micturition syncope), or who suffer from drops in blood pressure that cause dizziness when moving from a lying down or sitting position to sitting or standing (postural hypotension).
• This medicine is not recommended for people who have heart failure due to a mechanical obstruction of the heart, such as heart valve disease or blood clot in the lung (pulmonary embolism).
• This medicine is not recommended for children under 12 years of age.
• This medicine should not be used if you are allergic to one or any of its ingredients. Please inform your doctor or pharmacist if you have previously experienced such an allergy.

Pregnancy and breastfeeding

• This medicine should be used with caution during pregnancy and only if the potential benefit outweighs any potential risk to the developing baby. 
• This medicine may pass into breast milk in small amounts. It should only be used during breastfeeding if the benefit to the mother outweighs any potential risk to the nursing infant. 

Label warnings

• This medication may cause drowsiness. If affected do not drive or operate machinery.

Side effects

• Dizziness.
• Drowsiness.
• Headache.
• Feeling faint or fainting.
• Blurred vision.
• Balance problems involving the inner ear (vertigo).
• Awareness of your heartbeat (palpitations).
• Shortness of breath.
• Nasal congestion.
• Disturbances of the gut such as diarrhoea, constipation, nausea, vomiting or abdominal pain.
• Excessive fluid retention in the body tissues, resulting in swelling (oedema).
• Increased need to pass urine.
• Feeling weak.
• Depression.
• Skin reactions such as rash, sweating or itching.
• A drop in blood pressure that occurs when going from lying down to sitting or standing, which results in dizziness and lightheadedness (postural hypotension).
• Chest pain.
• Increased heart rate.
• Impotence.
• Persistent painful erection of the penis (priapism).

The side effects listed above may not include all of the side effects reported by the drug's manufacturer.

For more information about any other possible risks associated with this medicine, please read the information provided with the medicine or consult your doctor or pharmacist.

How can this medicine affect other medicines?

If prazosin is taken with other medicines that reduce blood pressure, either as a treatment for high blood pressure (antihypertensives), or as a side effect, there may be an enhanced blood pressure lowering effect. This might make some people feel dizzy, particularly when you first start taking the prazosin. If you do feel dizzy, you should lie down until the symptoms pass. If any dizziness persists you should let your doctor know, as your medicine doses may need adjusting. Other medicines that can reduce blood pressure include the following:

• ACE inhibitors, eg enalapril, captopril
• other alpha blockers, eg alfuzosin, doxazosin (this medicine is not normally recommended for use in combination with other alpha-blocker medicines)
• anaesthetics
• angiotensin II receptor antagonists, eg losartan
• antipsychotic medicines
• baclofen
• benzodiazepines, eg temazepam
• beta-blockers, eg propranolol
• calcium channel blockers, eg diltiazem, verapamil, nifedipine
• diuretics, eg furosemide
• levodopa
• MAOI antidepressants, eg phenelzine
• nitrates, eg glyceryl trinitrate, isosorbide mononitrate, amyl nitrate (poppers)
• phosphodiesterase type 5 inhibitors for impotence, eg sildenafil, vardenafil, tadalafil.

References:

https://www.pfizer.co.uk/prescription-medicines/hypovase

https://www.netdoctor.co.uk/medicines/heart-blood/a6883/hypovase-prazosin/

https://www.medicines.org.uk/emc/product/1068/smpc#gref

https://www.medicines.org.uk/emc/files/pil.1068.pdf

https://www.medicines.ie/medicines/hypovase-1mg-tablets-32407/patient-info

https://www.medicines.ie/medicines/hypovase-0-5-mg-tablets-32406/patient-infohttps://www.drugs.com/uk/hypovase-1mg-tablets-leaflet.html