Introduction The Dutch Pharmacogenetics Working Group (DPWG) indicated a list of actionable genotypes that affect patients’ response to more 50 drugs; these drugs which show variable effects based on patients’ genetic traits were named as pharmacogenetics (PGX) drugs. Preemptive genetic testing before using these drugs may protect certain patients from serious adverse reactions and could help in avoiding treatment failures. The objectives of this study include identifying the rate of PGX drug usage among Dutch population, estimating the level of users who carry the actionable genotypes and determining the main genes involved in drug’s effect variability. Methods Usage of PGX drugs over 2011–2017 by the insured population (an average of 11.4 million) in outpatient clinics in Netherlands was obtained from the publically available GIP databank. The data of 45 drugs were analyzed and their interactions with selected pharmacogenes were estimated. Frequency of actionable genotypes of 249 Dutch parents was obtained from the public database: Genome of Netherlands (GoNL), to identify the pattern of genetic characteristics of Dutch population. Results Over a 7 year period, 51.3 million exposures of patients to PGX drugs were reported with an average of 5.3 exposures per each drug user. One quarterof the exposures (12.4 million) are predicted to be experienced by individuals with actionable genotypes (risky exposures). Up to 60% of the risky exposures (around 7.5 million) were related to drugs metabolized by CYP2D6. SLCO1B1, and CYP2C19 were also identified among the top genes affecting response of drugs users (involved in about 22 and 12.4% of the risky exposures, respectively). Cardiovascular medications were the top prescribed PGX drug class (43%), followed by gastroenterology (29%) and psychiatry/neurology medications (15%). Women use more PGX drugs than men (55.8 vs. 44.2%, respectively) with the majority (84%) of users in both sexes are above 45 years. Conclusion PGX drugs are commonly used in Netherlands. Preemptive panel testing for CYP2D6, SLCO1B1, and CYP2C19 only could be useful to predict 95% of vulnerable patients’ exposures to PGX drugs. Future studies to assess the economic impact of preemptive panel testing on patients of older age are suggested.

Some patients prescribed flucloxacillin (~ 0.01%) develop drug-induced liver injury (DILI). HLA-B*57:01 is an established genetic risk factor for flucloxacillin DILI. To consolidate this finding, identify additional genetic factors, and assess relevance of risk factors for flucloxacillin DILI in relation to DILI due to other penicillins, we performed a genomewide association study involving 197 flucloxacillin DILI cases and 6,835 controls. We imputed single-nucleotide polymorphism and human leukocyte antigen (HLA) genotypes. HLA-B*57:01 was the major risk factor (allelic odds ratio (OR) = 36.62; P = 2.67 × 10 −97 ). HLA-B*57:03 also showed an association (OR = 79.21; P = 1.2 × 10 −6 ). Within the HLA-B protein sequence, imputation showed valine 97 , common to HLA-B*57:01 and HLA-B*57:03, had the largest effect (OR = 38.1; P = 9.7 × 10 −97 ). We found no HLA-B*57 association with DILI due to other isoxazolyl penicillins (n = 6) or amoxicillin (n = 15) and no significant non-HLA signals for any penicillin-related DILI.