Excessive bleeding has been reported in a few individuals taking ginkgo,^{10, 11} although a cause/effect relationship was not proven. In addition, two elderly individuals with well-controlled epilepsy developed recurrent seizures within two weeks after starting ginkgo.¹² Mild headaches lasting for a day or two and mild upset stomach have been reported in a small number of people using ginkgo.

Ginkgo leaves are known to contain a group of potentially toxic constituents known as alkylphenols. To reduce the potential for adverse effects, the German Commission E Monograph requires that ginkgo products for human consumption contain less than 5 parts per million of alkylphenols.¹³

One small clinical trial found that ginkgo supplementation for three months increased secretion of insulin by the pancreas, but did not affect blood glucose levels, in healthy young adults.¹⁴ These results suggest that the participants may have developed an insensitivity to insulin, a potential concern because insulin insensitivity may be a precursor to type 2 diabetes. However, this trial does not prove that ginkgo causes insulin insensitivity, nor does it prove that long-term ginkgo supplementation increases the risk for any disease. In addition, the results of this trial are not consistent with other research on ginkgo. Larger and more rigorously designed clinical trials of ginkgo supplementation have found no significant adverse effects after as many as 12 months of supplementation.¹⁵

People should seek an accurate medical diagnosis prior to self-prescribing ginkgo. This is especially important for the elderly, whose circulatory conditions can involve serious disease, and for people scheduled for surgery, as ginkgo may affect bleeding time.