I'm a big guy, but generally sensitive to substances. For me, 8 heaping teaspoons is usually enough. More can always be prepared if needed. It also depends on the quality of the kava. Some medium grind kava is little more than sawdust, while other kava can be potent. Experience will tell you the difference. If you drink too much you will become uncomfortably dizzy, so take it easy. Chasing the initial euphoria usually doesn't work unless you allow the kava to clear out somewhat first, which can take 2-3 hours. Pay attention to chemotypes as that will tell you what to expect in the way of headiness.