Ginkgo (Ginkgo biloba) is a well known herbal or natural medicine that is most commonly thought to be useful for memory, prevention and/or treatment of Alzheimer's related dementia, Raynaud's disease (or intermittent claudication), erectile dysfunction, multiple sclerosis and tinnitus to name a few.1  The wide variety of medical claims not only comes from anecdotal information, but also stems from some studies.  Furthermore, this variation in indications is also due to the various proposed mechanisms of action for ginkgo, which would have beneficial effects in each of these conditions. 

The active compounds found in ginkgo include flavonoids, bioflavonoids, proanthocyanidins, and trilactonic diterpenes (ginkgolide A, B, C).  The majority of these components contribute to one or more of its pharmacodynamic effects.   The proposed mechanisms of action for ginkgo include the following:2-11

While there are other possibly mechanisms, these reflect the main mechanisms that have some supporting pharmacodynamic data.  Unfortunately, some of this data is from animal or in-vitro analysis, thus making the extrapolation of information into humans with supporting clinically relevant outcomes more difficult.

References:

1. National Institutes of Health. National Center for Complementary and Alternative Medicine. Ginkgo.  NCCAM Publicaton No. D290. July 2010. 
2. Chung KF, Dent G, McCusker M et al.  Effect of a ginkgolide mixture (BN 52063) in antagonising skin and platelet responses to platelet activating factor in man.  Lancet  1987;1(8527):248-51.  
3. Rosenblatt M, Mindel J.  Spontaneous hyphema associated with ingestion of Ginkgo biloba extract.  N Engl J Med 1997;336:1108. 
4. Koch E.  Inhibition of platelet activating factor (PAF)-induced aggregation of human thrombocytes by ginkgolides: considerations on possible bleeding complications after oral intake of Ginkgo biloba extracts.  Phytomedicine  2005;12:10-6.  
5. Dorman DC, Cote LM, Buck WB. Effects of an extract of Gingko biloba on bromethalin-induced cerebral lipid peroxidation and edema in rats.  Am J Vet Res  1992;53:138-42.  
6. Otamiri T, Tagesson C.  Ginkgo biloba extract prevents mucosal damage associated with small-intestinal ischaemia.  Scand J Gastroenterol  1989;24:666-70. 
7. Logani S, Chen MC, Tran T et al. Actions of Ginkgo Biloba related to potential utility for the treatment of conditions involving cerebral hypoxia.  Life Sci  2000;67:1389-96.  
8. Barth SA, Inselmann G, Engemann R et al. Influences of Ginkgo biloba on cyclosporin A induced lipid peroxidation in human liver microsomes in comparison to vitamin E, glutathione and N-acetylcysteine.  Biochem Pharmacol  1991;41:1521-6.  
9. Duverger D, DeFeudis FV, Drieu K.  Effects of repeated treatments with an extract of Ginkgo biloba (EGb 761) on cerebral glucose utilization in the rat: an autoradiographic study.  Gen Pharmacol  1995;26:1375-83.  
10. Wu Y, Li S, Cui W et al. Ginkgo biloba extract improves coronary blood flow in healthy elderly adults: role of endothelium-dependent vasodilation.  Phytomedicine 2008;15:164-9. 
11. Liu HJ, Wang XL, Zhang L et al. Inhibitions of vascular endothelial growth factor expression and foam cell formation by EGb 761, a special extract of Ginkgo biloba, in oxidatively modified low-density lipoprotein-induced human THP-1 monocytes cells.  Phytomedicine  2009;16:138-45.  
12. Wada K, Ishigaki S, Ueda K et al.  An antivitamin B6, 4'-methoxypyridoxine, from the seed of Ginkgo biloba L.  Chem Pharm Bull  1985;33:3555-7. 
13. Arenz A, Klein M, Fiehe K et al. Occurrence of neurotoxic 4'-O-methylpyridoxine in Ginkgo biloba leaves, Ginkgo medications and Japanese Ginkgo food.  Planta Med  1996;62:548-51.  
14. Van Beek TA, Montoro P.  Chemical analysis and quality control of Ginkgo biloba leaves, extracts, and phytopharmaceuticals.  J Chromatogr A  2009;1216:2002-32.

Ginkgo, Ginkgo Biloba, Ginkgo Mechanism of Action