A 62-year-old woman presents with a 4-month history of worsening bilateral knee pain. The pain is worse on weight-bearing and at the end of the day, and is associated with stiffness for about 15 minutes in the morning. She has no systemic symptoms. Examination reveals crepitus on knee movement and mild bony enlargement. X-rays show joint space narrowing and osteophytes. What is the most appropriate first-line pharmacological management for her pain, according to NICE guidelines?