The ATP-binding cassette subfamily G member 2 (ABCG2), also known as breast cancer resistance protein (BCRP), is an efflux transporter expressed in key pharmacokinetic tissues and biological barriers. It regulates exposure to many endogenous compounds, drugs, and environmental toxins. Genetic variability in ABCG2 has been recognised as an important contributor to interindividual variability in drug response, especially in terms of efficacy and toxicity. This narrative review summarises current knowledge on the clinical relevance of ABCG2 genetic variants, with a focus on their effects on pharmacokinetics, adverse drug reactions and drug–drug–gene interactions, as well as their potential implementation in personalised therapy. A literature search was performed in PubMed, Scopus and the Clinical Pharmacogenomics Database (ClinPGx), with an emphasis on clinically relevant studies and available pharmacogenomic guidelines. The most investigated ABCG2 variant, c.421C>A (rs2231142; p.Gln141Lys), is consistently associated with reduced transporter activity and increased systemic exposure to several substrate drugs, including statins, allopurinol and anticancer agents, which may influence both treatment response and the risk of toxicity. Although growing evidence supports the clinical relevance of ABCG2 genotyping, its routine implementation remains limited. Integration of ABCG2 variability into polygenic models and clinical decision-support tools may further improve individualised treatment, particularly in patients with multimorbidity and polypharmacy.