Diclofenac is a nonsteroidal anti-inflammatory drug used to relieve mild to moderate pain associated with conditions like osteoarthritis and rheumatoid arthritis. It works by reducing substances that cause pain and inflammation. However, diclofenac can increase the risk of heart attack or stroke, especially with long term or high dose use or in those with heart disease. Common side effects include stomach issues like indigestion, bleeding, and water retention. Due to safety concerns, diclofenac should be used with caution in those with a history of stomach ulcers, kidney disease, or cardiovascular problems.