reach a turning point

To arrive at a point at which some event, activity, situation, condition, etc., begins to change in a dramatic way. I agreed to have a meeting with the mayor, unaware that I was about to reach a major turning point in my career. The medication is clearly not having the desired effect, so we're reaching a turning point in our treatment options. Either we try something much riskier, or else we transition the patient into palliative care.