According to WebMD Lion’s mane is possibly safe for most people when take appropriately for up to 16 weeks in a row. Side effects might be mild and may include stomach discomfort. However we had no problems using Lion’s mane extract powder for longer periods of time on a daily basis.
Take special precaution when pregnant and breast feeding because there is not enough reliable information about the safety of taking Lion’s mane during this time.
Also Lion’s mane might slow down blood clotting. This might increase the chances of bruising and bleeding in people with bleeding conditions. However, there are no reports of this occurring in humans, the studies were done just on mice. For humans this effect may actually have benefits, since it inhibits the production of excessive blood cells, which is usually the main reason for artery thickening. It is also useful for stopping blood clots from forming and thus encouraging your circulatory system to function at optimal levels.
It may also lower your blood sugar levels, so if you have diabetes monitor your blood sugar levels carefully, there are two studies done on mice and rats which show that using Lion’s mane extract lowers blood sugar levels almost by half and also it can increase you natural insulin level.
And the last warning, stop using Lion’s mane up to 2 weeks before your scheduled surgery, because it might cause extra bleeding during and after surgery.