As with all antipsychotic agents HALDOL has been associated with persistent dyskinesias. Tardive dyskinesia, a syndrome consisting of potentially irreversible, involuntary , dyskinetic movements, may appear in some patients on long-term therapy or may occur after drug therapy has been discontinued. The risk appears to be greater in elderly patients on high-dose therapy, especially females. The symptoms are persistent and in some patients appear irreversible. The syndrome is characterized by rhythmical involuntary movements of tongue, face, mouth or jaw (., protrusion of tongue, puffing of cheeks, puckering of mouth, chewing movements). Sometimes these may be accompanied by involuntary movements of extremities and the trunk.
Very common (10% or more): Extrapyramidal disorder (up to 34%), hyperkinesia (up to 13%), headache (up to 12%)
Common (1% to 10%): Tardive dyskinesia, dystonia, dyskinesia, akathisia, bradykinesia, hypertonia, somnolence, masked facies, tremor, dizziness, parkinsonism/parkinsonian effects
Uncommon (% to 1%): Convulsion, akinesia, cogwheel rigidity, sedation, involuntary muscle contractions, gait disturbance, persistent tardive dyskinesia
Rare (% to %): Motor dysfunction, neuroleptic malignant syndrome, nystagmus
Frequency not reported: Drowsiness, epileptic/grand mal seizure, vertigo, lethargy
Postmarketing reports: Opisthotonos [ Ref ]
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 .