# Current Uncertainties and Future Challenges Regarding NAD+ Boosting Strategies
## 4 Safety Dose of NAD+ Precursors  
WHO and the FAO recommend a daily intake of 11-12 mg of vitamin B3 (niacin), commonly known as nicotinic acid and nicotinamide, for adults [95]. NAD+ can be synthesized from the acid, amide, or riboside form of the vitamin B3 [96]. Based on numerous results from human clinical trials of nicotinic acid and nicotinamide, two bodies have established the highest daily dose for chronic ingestion that is unlikely to pose a risk of adverse health effects for almost everyone in the general population. The UK Expert Working Group on Vitamins and Minerals (EVM) [97] set a guideline for dietary supplementation with nicotinamide of 500 mg/day for a 60 kg adult, while the European Commission Scientific Committee on Food (EU SCF) set the tolerable upper intake level (UL) at 900 mg/day [98]. For preventing deficiency (pellagra), 16 mg of B3 for adult men and 14 mg for adult women are the recommended daily allowances (RDAs) [99]. Doses of nicotinic acid at 50 mg/day and higher are associated with flushing and itching that occur within 30 min after the oral administration [100]. Large doses of 250 mg per day of nicotinamide have induced reversible hepatotoxicity in animals and humans [101; 102] or minor abnormalities of liver enzymes [103]. Although nicotinic acid and nicotinamide have been studied for a very long time, there are some issues regarding the older studies. For example, according to [PERSON] and coworkers (2000), \"Safety data have not been collected in a systematic manner and many older reports failed to distinguish between nicotinamide acid and combined vitamin regimens containing nicotinamide\" [103]. NMN and NR on the other hand are relatively newly discovered forms of NAD+ precursors; consequently, there are fever studies on their toxicity and side effects. NR and NMN may be better tolerated compared to other nicotinamide adenine dinucleotide substrates since they do not trigger side effects, such as \"flushing\" or hepatotoxicity, which are characteristic for nicotinic acid, nor side effects of nicotinamide, including sirtuin inhibition [104]. No adverse effects (increased toxicity or increased mortality) were observed on wild-type mice after a 12-month administration of NMN, indicating its long-term safety [105]. A study assessing the safety of NR chloride at 300, 1000, and 3000 mg/kg/day over 14 and 90 days in rats revealed the lowest observed adverse effect level (LOAEL) for NR to be 1000 mg/kg/day, and the no observed adverse effect level (NOAEL) to be 300 mg/kg/day for NR [106]. The upper level (UL), which is by definition >>the maximum level of the daily nutrient intake that is likely to result in no adverse effect<< [99], was established for NR intake of 3 mg/kg/day (180 mg/day for a 60 kg adult). This was calculated from NOAEL by applying a 100-fold safety factor [106]. A few small human pharmacokinetics studies have been conducted on NR, primarily investigating pharmacokinetics in humans and the effects of treatment with NR on blood NAD+ levels. For example, NR was well-tolerated, as no severe side effects were reported by participants in two studies [107; 108; 109]. In a clinical trial with middle-aged human subjects, an oral dose of 1 g of NR once per day did not produce any drug-related adverse effects and clearly increased NAD+ levels (around 60% over the control group) in peripheral blood mononuclear cells [110].  
There are more Phase 1 clinical trials with NR than with NMN. The ClinicalTrials.gov database lists (on date 25 July 2022) 73 trials for nicotinamide riboside, 10 trials for nicotinamide mononucleotide, 619 trials for niacin (including nicotinic acid, niacinamide, and nicotinamide), and 290 trials for tryptophan (Table 1).  
The safety dose, therapeutic dose, therapeutic window, and optimal treatment duration for NAD+ precursors is still not known, especially in the case of NMN and NR. This information and the optimal therapeutic dose and treatment period of NAD+ precursors in various diseases should be determined in future human clinical trials.