Some drugs are administered in an inactive form called a pro-drug, like enalapril. It is the metabolite that is pharmacologically active, or enalaprilat in this case, which acts as an antihypertensive agent.
Some drug metabolites can be toxic such as those produced from paracetamol. These are usually detoxified by phase 2 conjugation joining with glutathione.
However, in an overdose situation where the dose of paracetamol is high, there is not enough glutathione to detoxify the metabolites. The metabolites accumulate causing toxicity and can result in hepatitis. As a solution, compounds are administration to boost the levels of glutathione so that phase 2 metabolism can take place, thus detoxifying the metabolites fully and reducing the risk of liver injury.