The key to the discovery of the working of sulphanilamide was found in 1940 when it was discovered that the inhibition of bacterial growth by sulphanilamide could be reversed by the addition of large quantities of p-amino benzoic acid (PABA). When it was further realised that PABA was an essential growth factor for certain bacteria, the effect could be explained as a competition between sulphanilamide and PABA.

Sulphanilamide and p- amino benzoic acid have many structural similarities. This results in them being able to interfere with each other when bonding with enzymes that are responsible for the synthesis of the vitamin folic acid in bacteria.

The folic acid (below) is built up, with the help of enzymes, by linking three molecular fragments:2 - amino – 4-hydroxy - 6 - methylpteridine (upper centre), L-glutaminic acid and p-amino benzoic acid. If sulphanilic acid instead of PABA binds onto the bonding-point on the enzyme, there is no resultant folic acid synthesis.

Humans require folic acid too, but they obtain it from food.