The adequate treatment of pain remains one of the major medical challenges. Morphine and other opioid drugs are most commonly used to counteract moderate to severe pain, but they are also increasingly accessed by patients with chronic non-malignant pain. To achieve long-term analgesia, opioid therapy still represents the standard treatment for chronic pain alleviation. This work presents an overview of current strategies aiming at controlled opioid release. Two important, and intrinsically linked, features are discussed in detail: the used formulations (i.e. polymer systems) and the applied drug administration routes. The different administration routes and their associated advantages and limitations are described. Links between the chemical structure of commonly used opioids and suited administration modes and formulations are made. This review can potentially give insight into new opportunities for adequate relief of chronic pain, a societal burden, by means of alternative (non-)opioid analgesics and may serve as inspiration for future developments in this area.
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