Alpha blocker

Alpha blockers (also called alpha-adrenergic blocking agents) constitute a variety of drugs which block α₁-adrenergic receptors in arteries and smooth muscles.

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Indications

These drugs may be used to treat:

• benign prostatic hyperplasia (BPH)
• high blood pressure (hypertension). This is not typically the drug of choice unless the patient also has BPH.
• symptoms of non inflammatory chronic pelvic pain syndrome, a type of prostatitis. As a side effect they may reduce blood pressure and result in lightheadedness.

Examples of alpha blockers

Alpha blockers include:

• Doxazosin (Cardura)
• Prazosin (Minipress)
• Phenoxybenzamine
• Phentolamine (Regitine)
• Tamsulosin (Flomaxtra/Flomax)
• Alfuzosin (Uroxatral)
• Terazosin (Hytrin)

Tamsulosin is relatively selective for α_1a-adrenergic receptors, which are mainly present in the prostate. Hence, it may have a more selective action in BPH with minimal effects on blood pressure.

Adverse effects and interactions

By reducing α₁-adrenergic activity of the blood vessels, these drugs may cause hypotension (low blood pressure) and interrupt the baroreflex response. In doing so, they may cause dizziness, lightheadedness, or fainting when rising from a lying or sitting posture (known as orthostatic hypotension or postural hypotension). For this reason, it is generally recommended that alpha blockers should be taken at bedtime. Additionally, the risk of first dose phenomenon may be reduced by starting at a low dose and titrating upwards as needed.

Because these medications may cause orthostatic hypotension, as well as hypotension in general, these agents may interact with other medications that increase risk for hypotension, such as other antihypertensives and vasodilators.

As discussed above, tamsulosin may have less risk for hypotension and orthostatic hypotension due to its selectivity for α_1a-adrenergic receptors. On the other hand, the drug (a) elevates risk for floppy iris syndrome, and (b) might show ADRs characteristic of the sulfa related drugs.

α₂-blockers

Although "alpha blocker" generally refers to α₁-blockers, there are also α₂-blockers, which are antagonists to the α₂ adrenergic receptor. Examples include yohimbine, benzylpiperazine, rauwolscine and idazoxan.

References

• DrugDigest - Alpha blockers
• RxList.com - Tamsulosin
• MeSH alpha-Adrenergic+Blockers