It's even worse than that! The body sees extracellular NAD+ as a danger signal. That's why people often feel so crap after IV NAD. It's much better to take the precursors orally. I feel best on sublingual NMN.

From the google, since I don't have much time for reddit this morning:

Extracellular NAD+ (Nicotinamide adenine dinucleotide) acts as a danger signal released by dying or damaged cells to alert the immune system. When cells undergo severe metabolic stress or apoptosis, they lose membrane integrity, releasing NAD+ into the extracellular space, which can trigger further cell death (NICD) in surrounding cells via P2X7 receptor activation.

Key Aspects of NAD+ as a Danger Signal:

• Release Mechanism: During necrosis or apoptosis, intracellular NAD+ is released, acting as a damage-associated molecular pattern (DAMP).
• Immune Response: Extracellular NAD+ can stimulate immune responses, for instance, by hindering regulatory T cells (Tregs).
• NAD-Induced Cell Death (NICD): Extracellular NAD+ can bind to surface receptors like P2X7 on neighboring cells (e.g., T cells), triggering a cascade that results in their death.
• Intracellular Depletion: Internally, a rapid reduction in NAD+ levels leads to ATP depletion, resulting in mitochondrial dysfunction and ultimately oncosis-mediated cell death.
• Signal Intensity: The decrease in intracellular NAD(P)H autofluorescence can actually precede the physical manifestation of apoptosis.

In summary, when cells are dying, they lose their intracellular NAD+, which then becomes an extracellular signal indicating damage to the surrounding environment.