Which of the following is a common side effect of long-term theophylline use?

Theophylline, a bronchodilator commonly used in the treatment of respiratory conditions such as asthma and chronic obstructive pulmonary disease (COPD), has several side effects that patients may experience with long-term use. Nausea and vomiting are particularly relevant as they are common gastrointestinal side effects associated with theophylline therapy. This may be due to the drug's effect on the gastrointestinal tract or its potential for causing increased stimulation of the central nervous system, leading to nausea.

Long-term use of theophylline does not typically lead to weight gain; instead, it can cause gastrointestinal irritation. Increased thirst is not a primary side effect attributed to theophylline; rather, increased urination and other renal effects can be observed. Weight loss may occur in some patients but is less commonly emphasized in literature concerning long-term theophylline use compared to nausea and vomiting. Therefore, the commonality of nausea and vomiting makes it the most appropriate choice regarding the side effects of long-term theophylline use.