Final answer:
Clients should be advised that anorexia, or loss of appetite, is a possible side effect when their pet is taking phenobarbital to control seizures.
Step-by-step explanation:
When advising clients who have pets taking phenobarbital to control seizures, it's important to inform them about potential side effects of this medication. Phenobarbital, a type of barbiturate, can lead to signs of overdose which include sluggishness, incoordination, difficulty in thinking, slow speech, faulty judgement, drowsiness, shallow breathing, staggering, and in severe cases, coma or death. These symptoms could be similar to the side effects seen with long-term use at therapeutic doses. The side effect most related to this question that a pet may exhibit while on phenobarbital is anorexia (B), which is a loss of appetite.