Various drugs are known to interact with haloperidol, a neuroleptic antipsychotic used in the treatment of schizophrenia and other forms of psychosis.

Some of the known drug interactions for haloperidol are described below:

• If haloperidol is used in combination with other central nervous system depressants such as sedatives, alcohol or narcotics, the actions and side effects of these drugs such as sedation and respiratory depression will be enhanced.
• The combined use of haloperidol and the blood pressure lowering medication methyldopa can increase the risk of extrapyramidal side-effects (relating to motor control) and other central nervous system effects.
• Haloperidol can decrease the effects of the treatment levodopa, which is used in Parkinson’s disease.
• Haloperidol disrupts the metabolism and elimination of tricyclic antidepressants. This increases the risk of the toxic effects associated with these drugs, which include anticholinergic and cardiovascular effects and the lowering of seizure threshold.
• Other medications that are also metabolized by the CY3A4 enzymatic pathway can also decrease or increase plasma levels of haloperidol. Quinidine, buspirone and fluoxetine increase the plasma haloperidol level while carbamazepine, phenobarbital and rifampicin decrease the plasma haloperidol level.
• The combined use of lithium and haloperidol has been shown to cause encephalopathy, extrapyramidal symptoms, coma and other neurological problems.
• Haloperidol decreases the action of guanethidine, a blood pressure lowering agent.
• When used in combination with adrenaline or epinephrine, haloperidol may cause a paradoxical decrease in blood pressure.