The Observed Safety Limit of glutamine supplementation, of which is the highest amount one can take and be assured of no side effects, has been suggested as being 14g/d in supplemental form (above food intake).  Higher levels than this have been tested and well tolerated, but there is not enough evidence to suggest that higher doses are completely free from harm over a lifetime of supplementation nor enough evidence to assume harm exists. Limited evidence suggests that 50-60g for a period of a few weeks is not associated with significant adverse effects.  
Rather than stimulating your body, niacin is able to unlock the energy that is available within the calories you consume. This means your body is able to achieve sustainable energy without any crash. While taking niacin pre workout supplements, niacin may also lead to flushing sensation which causes the skin tingle. When you realize this, known that this is normal and the effects will disappear after a few minutes. Just don’t worry if your pre-workout supplementation contains too much niacin. In fact, the more niacin, the better the results.