(Easy) Proton pump inhibitors, medlineplus.gov
(Medium) [3] Castellana, Chiara, et al. "Side effects of long-term use of proton pump inhibitors: practical considerations." POLSKIE ARCHIWUM MEDYCYNY WEWNETRZNEJ 131.6 (2021): 541-549.
(Difficult) [1] Ward, Robert M et al. "Proton pump inhibitors in pediatrics: mechanism of action, pharmacokinetics, pharmacogenetics, and pharmacodynamics." Pediatric Drugs 15 (2013): 119-131.
