Sublingual delivery solves the bioavailability problem
Molecules like NMN, NR and NAD⁺ that have low molecular weight and are hydrophilic can be absorbed through the capillaries under the tongue directly into the bloodstream. This is called Sublingual (under the tongue) delivery.
Sublingual delivery can bypass the stomach and liver.
This may solve the bioavailability problem of capsules that get digested in the stomach, so NAD⁺ can be used instead of NAD⁺ precursors. It also may improve the effectiveness of NMN.
Why no sublingual NR?
NR is not stable by itself, so Chromadex adds Chloride to make it stable.
ALL NR sold is actually Nicotinamide Riboside CHLORIDE
Unfortunately, the taste is not acceptable for sublingual use.
So NR is only available in capsule form, which much pass through the stomach and liver where it is metabolized to the less effective Nicotinamide (Liu, 2018) .