The goal of this clinical trial is to determine whether oral supplementation with different nicotinamide adenine dinucleotide (NAD) precursors can improve visual function in adults with primary open-angle glaucoma. The main questions it aims to answer are: 1. Does daily oral administration of equimolar doses of nicotinamide riboside (NR), nicotinamide (NAM), nicotinamide mononucleotide (NMN), or nicotinic acid (NA) improve visual field sensitivity in glaucoma patients over the short term? 2. How do plasma NAD+ metabolite profiles change after administration of each precursor, and do these changes relate to improvements in visual function? Researchers will compare NR, NAM, NMN, NA, and placebo groups to see if any of the NAD precursors lead to greater improvements in visual field sensitivity or changes in blood NAD+ metabolite levels compared to placebo. Participants will: Be randomly assigned to receive one of the four NAD precursors or placebo daily for two weeks. Undergo comprehensive eye examinations, including visual field testing and optical coherence tomography, at baseline and after two weeks. Provide blood samples before and after the intervention for measurement of NAD+ metabolites. Have safety monitored through clinical examination. This study will help identify whether boosting NAD+ levels with specific precursors offers functional benefit in glaucoma, and which blood metabolites may mediate these effects.