Therapeutic drug monitoring (TDM) is primarily used to ensure optimal drug concentrations in a patient's bloodstream, which directly impacts the effectiveness and safety of treatment. It helps in adjusting dosages to achieve the desired therapeutic effect while minimizing toxicity and adverse reactions. This is especially important for drugs with narrow therapeutic windows, significant variability in metabolism, or potential for severe side effects. By monitoring drug levels, clinicians can tailor therapy to individual patient needs, thereby improving clinical outcomes and preventing harm.

Reference: Goodman & Gilman’s The Pharmacological Basis of Therapeutics, Brunton et al., 13th Edition.