Formation of the metabolite starts once the drug is absorbed to the systemic circulation. This is assuming that the metabolite is not formed during the absorption of the parent drug.
Examine this part of the graph
Notice that the metabolite formation rate is slow initially then it increases gradually until it reaches its maximum rate when the drug concentration is at its maximum value. (Metabolite formation rate is dependent on the amount of the drug in the systemic circulation and the metabolite formation rate constant)
The metabolite conc-time profile will be described by a triexponential equation that depends on drug absorption, drug elimination (also metabolite formation), and metabolite elimination.
