Final answer:
The nurse should observe a client with bipolar disorder being treated with lithium for the common adverse effect of polyuria, which is increased urine output.
Step-by-step explanation:
When treating a client with bipolar disorder with lithium, one common adverse effect that nurses should be observant for is polyuria. Lithium can affect kidney function, leading to an increased output of urine. This can potentially lead to dehydration and electrolyte imbalances if not monitored and managed properly.
Therefore, it is crucial for healthcare providers to educate patients about the importance of maintaining adequate fluid intake and to monitor for signs of excessive urination.