It's common for the prostate gland to become enlarged as a man ages, this condition is called benign prostatic hyperplasia (BPH). As a man matures, the prostate goes through two main periods of growth. The first occurs early in puberty, when the prostate doubles in size. At around age 25, the prostate begins to grow again. This second growth phase often results in BPH years later.

Though the prostate continues to grow during most of a man's life, the enlargement doesn't usually cause problems until late in life. Severe BPH can cause serious prostate problems over time. Urine retention and strain on the bladder can lead to urinary tract infections, bladder or kidney damage, bladder stones, and incontinence.

The oil from the pumpkin seed is claimed to be useful in the management of BPH, and has been approved for this use by Germany’s Commission E. Phytosterols, or plant sterols, are a naturally occurring class of compounds called phytochemicals that are found in plants. Total sterol content in pumpkin seed kernel has been found to be high at 265 mg/100 g.

Researchers have examined the effect of pumpkin seed oil on testosterone-induced hyperplasia on the prostate of rats. Testosterone significantly increased prostate size ratio, and this induced increase was inhibited in rats fed with pumpkin seed oil at 2.0 mg/100 g of body weight. The protective effect of pumpkin seed oil was significant at the higher pumpkin seed oil dose. The study concluded that pumpkin seed oil can inhibit testosterone-induced hyperplasia of the prostate and could be beneficial in the management of BPH.

Therapeutic use and safety of a pumpkin seed extract has been tested in a multicentric clinical trial of 2,245 patients suffering from BPH. After receiving 1-2 pumpkin seed extract capsules per day for 12 weeks, it was concluded that pumpkin seed extract facilitates an efficient improvement of BPH symptoms with more than 96% of the patients reporting no undesired side effects.

In a 3-month randomized, double-blind study of 53 BPH patients, the preparation Curbicin, obtained from pumpkin seeds and dwarf palm plants was compared with a placebo in the treatment of BPH symptoms. Urinary flow, urination time, residual urine, frequency of urination and a subjective assessment of the effect of treatment were all significantly improved in the treatment group. No untoward side effects were noted.

In another study, researchers treated sixty patients in Stages I and II of BPH with PSO capsules. 26 of the patients took PSO for 10 months, 22 for at least 7, and 12 for at least 4 months. The daily dosage was 3 x 2 capsules in the first month and 3 x 1 capsules for the rest of the time. According to test results and changes in BPH symptoms, more than 80% of the patients experienced improvement. The PSO capsule therapy improved urinary flow, eased painful urination, and reduced the frequency of nocturnal urination.
References:
1. Phillips KM, Ruggio DM, Ashraf-Khorassani M. Phytosterol composition of nuts and seeds commonly consumed in the United States. J Agric Food Chem. 2005 Nov 30;53(24):9436-45.  PMID: 16302759.
2. Gossell-Williams M, Davis A, O'Connor N. Inhibition of testosterone-induced hyperplasia of the prostate of sprague-dawley rats by pumpkin seed oil. J Med Food. 2006 Summer;9(2):284-6. PMID: 16822218.
3. Friederich M, Theurer C, Schiebel-Schlosser G. Prosta Fink Forte capsules in the treatment of benign prostatic hyperplasia. Multicentric surveillance study in 2245 patients. Forsch Komplementarmed Klass Naturheilkd. 2000 Aug;7(4):200-4.  PMID: 11025395.  
4. National Kidney and Urologic Diseases Information Clearinghouse.
5. Carbin BE, Larsson B, Lindahl O. Treatment of benign prostatic hyperplasia with phytosterols. Br J Urol. 1990 Dec;66(6):639-41. PMID: 1702340.
6. Hamvas A, Corradi G, Hegedüs M, Frang D. Experience with the Peponen capsule in the management of benign prostatic hyperplasia. Int Urol Nephrol. 1991;23(1):51-5. PMID: 1718920.