1. Introduction

Pain relief, both for acute and chronic pain, is an important aspect of modern medicine and healthcare services [1]. Pain is defined as 'an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage' by the International Association for the Study of Pain (IASP) [2]: as such, it strongly worsens the quality of life of patients and remains one of the most common reasons for using health care services [1]. Pain relief drugs in the form of analgesics represent one of the most commonly used medications. While analgesics may include different therapeutic active compounds,

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covering specific pain relief needs, opioid medications and nonsteroidal anti-inflammatory drugs (NSAIDs) remain the most commonly used analgesics [1, 3, 4].

Pain relief management teams have to be aware of the exact nature of the pain itself and its intensity, and must be able to differentiate between acute and chronic pain. These are not only essential factors for pain management itself, but this could also lead to the consideration of possible adverse drug reactions (ADRs) from analgesic use. Indeed, when implementing pain relief management, one must consider the best practice to alleviate pain directly, along with the possibility of having ADRs which would then defeat the purpose of analgesics use itself [5–8]. Additionally, chronic pain is associated with an increased incidence of mental health issues such as anxiety and depression [9]; thus, there is a need to extend the consequences of inefficient pain relief beyond pain management alone.

The proper use of analgesics, that is, targeted drug use against specific types of pain, can avoid or at least minimize ADRs. In this regard, scientific studies reporting on ADRs caused by analgesics become an invaluable tool to predict and prevent ADRs and to evaluate the safety of analgesics in different pain relief practices. While short-term side effects are generally easier to observe, long-term effects, particularly in chronic analgesic users, need specially designed studies or a careful review of previous literature. In the last few years, more literature has been made available that addresses ADRs of both the opioid and NSAID type, allowing for the re-evaluation of the safety of these two medication classes, including their chronic long term use [10–16].

In order to draw attention to analgesic an their risks and to minimize the negative consequences related to their use, the present review comprises a synthesis of the most important safety issues described in scientific literature. This stands as a broad overview of the topic, providing a basic understanding of safety issues associated with analgesics and a starting point for further understanding of the subject at hand.

The ADRs associated with the two most commonly used analgesic classes, opioid and nonsteroidal anti-inflammatory drugs (NSAID), discussing their common adverse effects and how these can influence their usability in clinical applications. In recent years, more and more longterm studies have been published providing an insight into the potential risks of long-term analgesic use, this chapter provides a thorough overview. This is particularly important when discussing opioid analgesics, whose chronic use can lead to analgesic tolerance and even addiction. A full description of the potential problems derived from analgesic use represents the first step in optimizing protocols for its safe application in clinical settings.
