A client suffering from asthma is placed on theophylline to reduce the frequency of acute attacks. Which of the following adverse effects would the nurse caution the client prior to discharge?

A. Tachycardia: Theophylline can stimulate the heart, leading to an increase in heart rate. Tachycardia is a common adverse effect associated with theophylline use, and it is essential for the client to be aware of this potential side effect, especially if they have underlying heart conditions. B. Constipation: While gastrointestinal side effects can occur with theophylline, constipation is not a primary or common adverse effect. Theophylline may actually lead to gastrointestinal upset or increased gastric acid production rather than causing constipation. C. Drowsiness: Theophylline typically does not cause drowsiness. In fact, it is more likely to cause restlessness or insomnia, as it is a stimulant. Thus, advising the client about drowsiness is not relevant in this case. D. Oliguria: Oliguria (reduced urine output) is not a common adverse effect of theophylline. Theophylline can affect kidney function indirectly but does not typically present as oliguria. Monitoring for any renal changes is essential, but oliguria is not a primary concern.