Drug metabolism

Drug metabolism chemically transforms a drug into other molecular forms. Oxidation, hydrolysis and conjugation are common routes, frequently involving the liver but also other tissues. Transformation may inactivate a compound, generate an active product or activate a precursor; metabolism therefore does not mean detoxification in every case. ^[ref]

The parent compound and each product have their own time courses. In a simple pathway $D\to M\to$ further products, a fall in $D$ can coexist with a rise in an active metabolite $M$. Measuring only the parent can therefore misrepresent pharmacologically relevant exposure. Chemical conversion, excretion of unchanged material and movement into a different tissue are distinct processes.