This information is for men who have an enlarged prostate. It tells you about 5-alpha-reductase inhibitors, a treatment used for an enlarged prostate. It is based on the best and most up-to-date research.
Do they work?
Yes. Drugs called 5-alpha-reductase inhibitors should help your symptoms, especially if your prostate is very large. They may shrink your prostate or stop it getting any bigger. But they can take a long time to work. You may not notice an improvement for four months to six months. However, once the drug starts working, it can work for several years, as long as you keep taking it.
5-alpha-reductase inhibitors reduce your chances of suddenly being unable to urinate. They are the only treatment for an enlarged prostate that can do this. Suddenly being unable to urinate is called acute urinary retention and needs emergency treatment.
What are they?
5-alpha-reductase inhibitors are also called anti-androgen therapy or hormone therapy.
As the name 5-alpha-reductase inhibitor suggests, these drugs work by blocking the effects of 5-alpha-reductase. 5-alpha-reductase is a kind of protein called an enzyme. Your prostate needs this enzyme to grow.
There are two drugs of this type used to treat an enlarged prostate. They are (with brand names):
You can also get a drug called Combodart, which is a capsule that contains dutasteride and a drug called tamsulosin (an alpha blocker).
How can they help?
Taking one of these drugs can shrink your prostate gland, which should make it easier for you to urinate.    You may find:
You have a stronger flow of urine
You have to strain or push less to urinate
You don't have to wait so long before your urine starts to flow
You don't have to rush to the toilet so much
You no longer have the sensation that your bladder isn't quite empty after you urinate.
After three to six months of taking finasteride, your prostate can get about a third smaller. After this, your prostate won't shrink any more, but it won't grow again for at least four years if you keep taking the drugs. 
Taking finasteride for four years can halve your chances of suddenly being unable to pass urine (acute urinary retention) and needing prostate surgery.  Acute urinary retention is a serious condition that needs fast treatment. To read more, see Complications of an enlarged prostate.
We don't know if finasteride works better than dutasteride. There are no studies comparing the two drugs.
We also don't know for certain how 5-alpha-reductase inhibitors compare with drugs called alpha-blockers. Two studies found that an alpha-blocker called tamsulosin improved symptoms more than finasteride after four weeks and 12 weeks of treatment, but not after 24 weeks.   Other research found that dutasteride worked better than tamsulosin after 24 months.