Alpha-Blockers for Prostate Enlargement: Some Important Drug Interactions

Alpha-blockers are often used to lower blood pressure or to treat enlarged prostates.

However, taking alpha-blockers in combination with drugs for erectile dysfunction and with other drugs can cause dizziness and fainting.

In this article we will discuss alfuzosin (UROXATRAL), doxazosin (CARDURA), tamsulosin (FLOMAX) and terazosin (HYTRIN).

Interactions common to all alpha-blockers

With blood pressure medications. Alpha-blockers have the ability to cause a sudden drop in blood pressure, especially at the beginning of therapy. This is why patients are instructed to start with low doses of these drugs and take precautions, whether or not they are using the drugs with other blood pressure-lowering medications.

With other alpha-blockers. Little is known about the effects of combining more than one alpha-blocker, and the manufacturers therefore state that such combinations should not be used.

Interactions of specific alpha-blockers with other drugs

Tamsulosin (FLOMAX). Tamsulosin is metabolized by the enzymes CYP2D6 and CYP3A4, but not much information is available on combining tamsulosin with drugs that inhibit either of these enzymes. Nonetheless, the product information recommends that tamsulosin be used with caution in people also receiving moderate to strong inhibitors of CYP2D6 or CYP3A4, especially if the tamsulosin dose is greater than 0.4 mg daily. From a practical standpoint, this means that when any new drug is added to your regimen, it would be prudent to watch for excessive tamsulosin effects.

Some well-known inhibitors of CYP2D6 include fluoxetine (PROZAC), paroxetine (PAXIL), diphenhydramine (BENADRYL), amiodarone (CORDARONE, PACERONE) and haloperidol (HALDOL). For inhibitors of CYP3A4, see the section on alfuzosin below.

Using tamsulosin with the ulcer drug cimetidine (TAGAMET) has been shown to increase tamsulosin blood levels by 44 percent, so patients should use an alternative to cimetidine such as famotidine (PEPCID), nizatidine (AXID) or ranitidine (ZANTAC) instead, since they are unlikely to interact.

Terazosin (HYTRIN). There is evidence that terazosin may have additive hypotensive (blood pressure-lowering) effects when combined with drugs used for erectile dysfunction, such as sildenafil (VIAGRA), tadalafil (CIALIS) or vardenafil (LEVITRA). If you are taking terazosin and also use erectile dysfunction drugs, be alert for evidence of hypotension (such as lightheadedness and/or fainting).

Doxazosin (CARDURA). Two studies found that the erectile dysfunction drug tadalafil increased the hypotensive effects of doxazosin in some people, and it seems likely that other erectile dysfunction drugs would act similarly. If you are taking doxazosin and also use erectile dysfunction drugs, be alert for evidence of hypotension (such as lightheadedness and/or fainting).

Alfuzosin (UROXATRAL). Alfuzosin has more drug interactions than the other alpha-blockers. It is metabolized primarily by CYP3A4, and potent CYP3A4 inhibitors may produce several-fold increases in alfuzosin blood levels. Accordingly, the product information states that potent CYP3A4 inhibitors such as the antifungals ketoconazole (NIZORAL) and itraconazole (SPORANOX), and the antiretroviral drug ritonavir (NORVIR) should not be taken with alfuzosin.

Other CYP3A4 inhibitors that are likely to interact with alfuzosin (UROXATRAL) include the following: