Haloperidol is a typical butyrophenone type antipsychotic that exhibits high affinity dopamine D 2 receptor antagonism and slow receptor dissociation kinetics.  It has effects similar to the phenothiazines .  The drug binds preferentially to D 2 and α 1 receptors at low dose (ED 50 = and mg/kg, respectively), and 5-HT 2 receptors at a higher dose (ED 50 = mg/kg). Given that antagonism of D 2 receptors is more beneficial on the positive symptoms of schizophrenia and antagonism of 5-HT 2 receptors on the negative symptoms, this characteristic underlies haloperidol's greater effect on delusions, hallucinations and other manifestations of psychosis.  Haloperidol's negligible affinity for histamine H 1 receptors and muscarinic M 1 acetylcholine receptors yields an antipsychotic with a lower incidence of sedation, weight gain, and orthostatic hypotension though having higher rates of treatment emergent extrapyramidal symptoms .
The dose of haloperidol decanoate injection 50 mg/mL or haloperidol decanoate injection 100 mg/mL should be expressed in terms of its haloperidol content. The starting dose of haloperidol decanoate should be based on the patient's age, clinical history, physical condition, and response to previous antipsychotic therapy. The preferred approach to determining the minimum effective dose is to begin with lower initial doses and to adjust the dose upward as needed. For patients previously maintained on low doses of antipsychotics (., up to the equivalent of 10 mg/day oral haloperidol), it is recommended that the initial dose of haloperidol decanoate be 10 to 15 times the previous daily dose in oral haloperidol equivalents; limited clinical experience suggests that lower initial doses may be adequate.
Benztropine and diphenhydramine are used to treat acute dystonia, a known side effect of typical antipsychotics such as haloperidol.
Acute dystonia has the quickest onset of any of the known extrapyramidal side effects of typical antipsychotics, typically between four hours and four days. It is characterized by sustained muscle contractions causing twisting and repetitive movements or abnormal postures. Anticholinergics and antihistamines are commonly used in treatment. While benztropine is used to treat all extrapyramidal side effects (Answers 1-4), acute dystonia has the quickest onset of the listed side effects and is therefore the correct answer choice given the timeframe mentioned in the question.
Dilsaver reviews antipsychotic agents. High-potency antipsychotics such as haloperidol have a high affinity for dopamine receptors and are thus more likely to cause extrapyramidal side affects than lower potency agents. Low-potency agents, such as chlorpromazine, have a higher affinity for muscarinic and alpha adrenergic receptors.
Dressler et al. review acute movement disorders. Drug-induced movement disorders are almost always caused by dopamine receptor blockers and often affect the cranial pharyngeal or cervical muscles. Anticholinergics are widely used as treatment because they are quick in onset.
Figure A illustrates a possible position assumed with sustained muscle contraction in dystonia.
Illustration A diagrams the common dosages and side effect profiles of commonly used antipsychotics, including each drug’s propensity to cause extrapyramidal side effects.
Answer 2: Akinesia is an extrapyramidal side effect that commonly occurs after approximately 4 days of treatment with typical antipsychotics.
Answer 3: Akasthisia is an extrapyramidal side effect that commonly occurs after approximately 4 weeks of treatment with typical antipsychotics.
Answer 4: Tardive dyskinesia is an extrapyramidal side effect that commonly occurs after approximately 4 months of treatment with typical antipsychotics.
Answer 5: Seizures are a known side effect of clozapine, an atypical antipsychotic.