 There is disagreement amongst researchers regarding the amount of lactose needed to cause symptoms in those who are lactose intolerance. The general consensus is that the amount of lactose in a glass of milk 12 grams is enough to cause mild symptoms in most patients who are lactose intolerant 1. However, there have been a number of studies and case studies that argue that much lower amounts can cause symptoms 2, 3, 4, 5. This could be as little as 0.02 grams 6. 