Final answer:
St. John's wort can decrease theophylline's serum drug levels by inducing the liver enzyme cytochrome P450 and increasing theophylline's metabolism.
Step-by-step explanation:
The herbal product that can decrease theophylline's serum drug levels when taken together is St. John's wort. Theophylline is a methylxanthine drug with therapeutic effects such as stimulating the central nervous system and relaxing smooth muscles. While caffeine is also a methylxanthine and shares similar structures and pharmacological actions with theophylline, it's not known to significantly decrease theophylline's serum levels.
Instead, St. John's wort induces the activity of the liver enzyme cytochrome P450, which is responsible for the metabolism of many drugs, including theophylline. As a result, this concurrent use can lead to decreased serum levels of theophylline, potentially reducing its effectiveness. Patients should be advised to avoid taking St. John's wort with theophylline to maintain appropriate therapeutic drug concentrations.