<p>Nicotinamide adenine dinucleotide (NAD⁺) is an essential molecule involved in cellular metabolism, and its decline has been implicated in ageing and age-related disorders. However, evidence for an age-related decline in NAD⁺ levels in humans has been consistently observed only in a limited number of studies. Similarly, although preclinical studies support the idea that supplementation with NAD⁺ precursors is a promising therapeutic strategy to promote healthy ageing, human clinical trials have shown limited efficacy. Therefore, an increasing understanding of how NAD⁺ metabolism is affected in different tissues during disease and following NAD⁺ precursor supplementation is crucial to defining the therapeutic value of NAD⁺-targeted therapies. In this Review, we evaluate the clinical evidence supporting the notion that NAD⁺ levels decline with age, as well as the tissue-specific effects of NAD⁺ precursor supplementation. Viewed in perspective, the published body of data on NAD⁺ dynamics in human tissues remains sparse, and the extrapolation of rodent-based data is not straightforward, underscoring the need for more clinical studies to gain deeper insights into systemic and tissue-specific NAD⁺ metabolism.</p>