Monoclonal antibodies against tumour necrosis factor (TNF) and other cytokine targets fail in a predictable fraction of patients with inflammatory bowel disease (IBD) and inflammatory arthritis, and a large share of that failure is pharmacokinetic. Therapeutic drug monitoring (TDM) measures trough drug concentration and anti-drug antibodies (ADA) so that dosing can be corrected. Reactive TDM, performed when response is lost, is supported by randomised and observational evidence and by every major guideline. Proactive TDM, in which levels are checked in stable patients and doses adjusted to a target, is contested. This review appraises the randomised trials in both specialties and their observational precursors. In infliximab maintenance, the NOR-DRUM B trial (n=458) reported sustained disease control in 73.6% of patients under proactive TDM against 55.9% under standard care, and a 2024 meta-analysis estimated a relative risk of 1.26 (95% CI 1.14 to 1.40) for this setting, rated low certainty. Proactive dosing during induction (TAILORIX, NOR-DRUM A) and adult adalimumab maintenance (SERENE CD) showed no benefit, whereas paediatric adalimumab (PAILOT) and Bayesian infliximab dosing (PRECISION) favoured concentration-guided care. Rheumatology evidence centres on avoiding overexposure: adalimumab response plateaus at 5 to 8 mg/L, and interval extension above 8 µg/mL proved non-inferior. Proposed thresholds, their derivations and their limits are tabulated. Pharmacist-run programmes in Australia and Spain have tripled the proportion of patients in the target range and reduced surgery and hospitalisation against historical controls. A pharmacist-led pathway is proposed for Indian hospitals, where cost and turnaround currently restrict TDM to a minority of centres. Proactive monitoring is justified for maintenance infliximab; elsewhere, reactive TDM with structured interpretation remains the defensible standard.