Proton pump inhibitors (PPIs) are the undisputed leaders in the treatment of acid-related diseases. In actual clinical practice, the use of PPIs has been growing exponentially in recent decades. For a long time, PPIs were considered completely safe drugs for both short-term and long-term use. However, modern clinical guidelines and reviews and meta-analyses of the current decade on the safety of PPI use note that when prescribing PPIs in large doses for a long period, the possibility of developing numerous side effects should be taken into account. We searched the PubMed and Scopus information databases for publications on the safety of PPI use, including sources up to 12/01/2023. The review addresses issues of drug interactions between PPIs and chemotherapeutic drugs, which should certainly be taken into account in real clinical practice. In addition, recent reviews and meta-analyses have examined side effects of long-term PPI use, such as increased risk of infection (Clostridium difficile infection, community-acquired pneumonia), electrolyte disturbances, kidney damage, increased risk of hip, wrist and spine fractures; increased cardiovascular risk and a number of others, which can also have an adverse effect on the prognosis of cancer patients. The risk of developing gastric cancer with long-term PPI therapy can be considered as a likely side effect, since their association with PPI use is not denied or questioned in recent reviews and meta-analyses. There is still uncertainty regarding the increased risk of other cancers, since there are publications that do not confirm their association with PPI use. However, the presented review, despite a number of contradictions, allows us to conclude that the increased cancer risk with long-term use of PPIs is real and must be taken into account when prescribing PPIs for a long term.