While both are referring to the same molecule, effervescent indicates a specific delivery method which can improve absorption and just be easier for some folks to use.

Differences: N-acetylcysteine is formulated as both IV or inhalation solutions, whereas the effervescent formulation (brand name Legubeti, FDA approved Feb 2024) is given as an oral solution. According to the prescribing information, Legubeti active ingredient is actually acetylcysteine lysine, but is still dosed based on acetylcysteine. Dosing is technically not an exact conversation (4.74 g of acetylcysteine lysine = 2.5 g acetylcysteine), however, clinically, we are always dosing based on acetylcysteine. It appears to be manufactured in two strengths 500mg and 2.5 g (acetylcysteine content).

Tldr: nac or acetylcysteine lysine --->convert to acetylcysteine. We always dose based on acetylcysteine. The effervescent is specifically formulated for PO administration and indicated for acetaminophen OD.