Background: Diclofenac is the most often prescribed non-steroidal anti-inflammatory drug worldwide. It is a phenylacetic acid derivative that works by inhibiting cyclooxygenase enzyme and has analgesic, antipyretic and anti-inflammatory activities. Like other non-steroidal anti-inflammatory drugs, it is sold over the counter without any control over the use or the dose, and although it is an effective drug with many useful effects, with its inhibition of prostaglandin biosynthesis, harmful effects occur in both animals and humans. Aim: The current paper aims to gather information on diclofenac regarding its chemical composition, mechanism of toxicity and harmful effects. Conclusion: In summary, research proved that diclofenac has multiple toxicological effects affecting many organs and it should be used in a controlled and supervised manner.