Bitter melon possesses significant hypoglycemic effect. The highest oral hypoglycemic activity and the maximum improvement of the oral glucose tolerance were associated with the extract of Momordica charantia (bitter melon).

– J Ethnopharmacol 1984 Jul;11(2):223-31 — Oral hypoglycaemic activity of some medicinal plants of Sri Lanka. — Karunanayake EH, Welihinda J, Sirimanne SR, Sinnadorai G.

A water-soluble extract of the fruits significantly reduced blood glucose concentrations during a 50 g oral glucose tolerance test in diabetics. Doctors supervising diabetics should be aware of the fruit’s hypoglycemic properties.

– Br Med J (Clin Res Ed) 1981 Jun 6;282(6279):1823-4 — Improvement in glucose tolerance due to Momordica charantia (karela). — Leatherdale BA, Panesar RK, Singh G, Atkins TW, Bailey CJ, Bignell AH.

High doses or highly concentrated usage may have adverse effects on general health.
– Eur J Cancer Prev 2000 Aug;9(4):283-8 — Prevention of carcinogen-induced mouse skin papilloma by whole fruit aqueous extract of Momordica charantia. — Ganguly C, De S, Das S.

Four clinical trials found bitter melon juice, fruit, and dried powder to have a moderate hypoglycemic effect. Reported adverse effects of bitter melon include hypoglycemic coma and convulsions in children, reduced fertility in mice, a favism-like syndrome, increases in gamma-glutamyltransferase and alkaline phosphatase levels in animals, and headaches. Bitter melon may have additive effects when taken with other glucose-lowering agents.
– Am J Health Syst Pharm 2003 Feb 15;60(4):356-9 — Bitter melon (Momordica charantia): a review of efficacy and safety. — Basch E, Gabardi S, Ulbricht C.

The pharmacology, clinical efficacy, adverse effects, drug interactions, and place in therapy of bitter melon are described. Bitter melon (Momordica charantia) is an alternative therapy that has primarily been used for lowering blood glucose levels in patients with diabetes mellitus. Components of bitter melon extract appear to have structural similarities to animal insulin. Antiviral and antineoplastic activities have also been reported in vitro. Four clinical trials found bitter melon juice, fruit, and dried powder to have a moderate hypoglycemic effect. These studies were small and were not randomized or double-blind, however. Reported adverse effects of bitter melon include hypoglycemic coma and convulsions in children, reduced fertility in mice, a favism-like syndrome, increases in gamma-glutamyltransferase and alkaline phosphatase levels in animals, and headaches. Bitter melon may have additive effects when taken with other glucose-lowering agents. Adequately powered, randomized, placebo-controlled trials are needed to properly assess safety and efficacy before bitter melon can be routinely recommended. Bitter melon may have hypoglycemic effects, but data are not sufficient to recommend its use in the absence of careful supervision and monitoring.

– Bitter melon (Momordica charantia): a review of efficacy and safety. — Basch E, Gabardi S, Ulbricht C. — Am J Health Syst Pharm. 2003 Feb 15;60(4):356-9.