What is the preferred medication for a patient displaying involuntary facial muscle contractions after starting haloperidol?

The correct choice for managing involuntary facial muscle contractions, often associated with the use of haloperidol, is diphenhydramine. Haloperidol, an antipsychotic, can cause extrapyramidal symptoms, including tardive dyskinesia and acute dystonic reactions, which can manifest as involuntary muscle contractions. Diphenhydramine, an antihistamine with anticholinergic properties, is effective in treating these symptoms. It helps to counteract the effects of dopamine blockade caused by haloperidol, thereby relieving muscle contractions.

Lorazepam, while it may provide some benefit in terms of sedation and anxiety relief, is not the preferred choice for treating acute dystonic reactions. It can be useful in managing anxiety or agitation, but does not specifically target the muscle contractions caused by the medication.

Phenobarbital is a barbiturate that has sedative properties, but it is not indicated for the management of extrapyramidal symptoms and would not effectively address the muscle contractions resulting from haloperidol.

Metoprolol is a beta-blocker used primarily for cardiovascular issues and does not have a role in the management of involuntary muscle contractions or the extrapyramidal side effects associated with antipsychotic medications.