The file
Four syllables about the sound of two tablets hitting water, written by a guy who'd had a Top 5 hit in 1966. Tom Dawes moved from pop to jingles and this was the one that outlived the band.
Alka-Seltzer's ads were the best in the business in the '70s, and this tune is the part everyone kept. The spots with the guy who couldn't believe he ate the whole thing are the ones parents quoted.
Sources: Wikipedia — Alka-Seltzer