What are the maximum doses for N-acetylcysteine (NAC), Nicotinamide adenine dinucleotide (NAD), Glycine, Pyrroloquinoline quinone (PQQ), and L-Tyrosine?

Maximum Doses for NAC, NAD, Glycine, PQQ, and L-Tyrosine

N-Acetylcysteine (NAC)

For acetaminophen overdose, the maximum loading dose is 140 mg/kg orally, followed by maintenance doses of 70 mg/kg every 4 hours for 17 additional doses. 1

• The FDA-approved dosing for acetaminophen toxicity uses body weight-based calculations, with loading doses reaching up to 15 grams (for patients 100-109 kg) and maintenance doses up to 7.5 grams per dose 1
• In clinical practice for other indications (antioxidant/mucolytic use), oral NAC is commonly used at doses ranging from 600-1800 mg daily, though the compound has a well-established safety profile even at higher doses 2, 3
• NAC toxicity is uncommon and dose-dependent, with the route of administration affecting tolerability 3

NAD (Nicotinamide Adenine Dinucleotide) Precursors

Baseline requirements for NAD+ synthesis can be met with less than 20 mg daily of niacin (nicotinic acid and/or nicotinamide), though substantially higher doses may provide additional benefits. 4

• The recommended dietary allowance for niacin is under 20 mg daily for basic NAD+ synthesis, but emerging evidence suggests higher rates of NAD+ synthesis may be beneficial for neuroprotection 4
• Nicotinamide riboside, a newer NAD+ precursor, shows promise for supporting neuronal NAD+ synthesis specifically, though optimal human supplementation doses require further research 4
• No specific maximum dose is established in the provided guidelines, but therapeutic use typically remains well below gram quantities

Glycine

No specific maximum dose for glycine supplementation is provided in the available clinical guidelines or FDA drug labels.

• The evidence provided does not contain glycine-specific dosing information from authoritative sources
• Glycine is a non-essential amino acid with generally recognized safety, but maximum therapeutic doses should be determined based on specific clinical context and individual patient factors

PQQ (Pyrroloquinoline Quinone)

No maximum dose for PQQ is established in FDA drug labels or major clinical practice guidelines.

• The provided evidence does not include PQQ-specific dosing recommendations from guideline societies or regulatory agencies
• PQQ is not currently regulated as a pharmaceutical agent with established maximum doses in the reviewed literature

L-Tyrosine

N-acetyl-L-tyrosine (NAT) in parenteral nutrition at approximately 1 g total amino acids/kg/day provides sufficient aromatic amino acid content to meet recommended dietary allowances. 5

• In parenteral nutrition formulations using Aminosyn II 15%, approximately 35% of administered NAT is excreted unchanged in urine, but sufficient amounts are retained to meet aromatic amino acid requirements 5
• When NAT is infused intravenously at 5 grams over 4 hours in healthy volunteers, plasma tyrosine levels increase by only 25%, with 56% of NAT excreted unchanged in urine, suggesting limited conversion efficiency 6
• The usefulness of NAT as a tyrosine precursor during intravenous infusion is questionable based on metabolic studies, as no net production of tyrosine was found from the splanchnic area 6

Important Caveats

• NAC is the only compound among these five with well-established maximum doses in FDA-approved drug labeling, specifically for acetaminophen overdose 1
• For NAD precursors, glycine, PQQ, and L-tyrosine used as dietary supplements rather than pharmaceuticals, maximum safe doses are not definitively established in major clinical guidelines 4, 5
• The conversion efficiency of acetylated amino acid precursors (NAT, NAC) to their parent amino acids is limited, with substantial urinary excretion of unchanged compounds 6, 5