It is possible to overdose on niacinamide http://www.livestron...amide-overdose/ (laymen's summary on the subject, There is also one in wikipedia with references to journals)The overdose amount is around three grams. Since niacinamide is nicotinamide (same molecule, different names) then nicotinamide riboside (NR) could theoretically be overdosed. As niacinamide accounts for roughly half the weight of the NR molecule, more than 6 grams would possibly cause the symptoms of niacinamide toxicity, which include nausea, dizziness and falls, bleeding ulcers and so forth.
So this seems to imply an upper limit on NR supplementation, unless different pathways are involved.....