What is the mechanism of action for Theophylline? What are some adverse effects of theophylline?Explain

Theophylline is a medication that is less commonly used in treating COPD but can be an option for some patients. Theophylline is a xanthine derivative and is not considered a first line medication for COPD patients (Woo & Robinson, 2016).
What is the mechanism of action for Theophylline?
What are some adverse effects of theophylline?