Saccharin.

Aspartame is clean but slow and unstable at low pH over time, while saccharin is fast, stable and leaves a metallic finish. Together they cover the full sweetness curve and each is used below the level where its own off-note registers. The blend is formulation economics and taste engineering, not a physiological interaction.

A small amount of sucrose alongside saccharin gives body and mouthfeel that a high-intensity sweetener cannot supply on its own, and the perceived sweetness exceeds what either concentration would give alone. Reduced-sugar products use this to cut sucrose without a hollow taste. The relationship is sensory.

Zinc salts, particularly in lozenges and liquids, carry a strong metallic astringency that stops people finishing a course. Saccharin is used as one of the masking agents in those preparations. Worth noting that saccharin has its own metallic tail, so it is usually combined with a second sweetener when masking a mineral.

Rodent and laboratory work has reported that saccharin inhibits growth of some bacterial species, while a human trial at high intake did not detect microbiota changes. Whether co-ingesting saccharin affects a live probiotic dose in people has not been established either way. Read this as a theoretical concern flagged by animal data rather than a demonstrated human interaction.

Effervescent bases taste salty and alkaline, and saccharin holds its sweetness across the pH swing during dissolution where some other sweeteners degrade. That heat and pH stability is why it persists in effervescent formats. The pairing is about surviving the dosage form.