Haldol is in the classification of antipsychotic meds....like Thorazine and Mellaril. These three are some of the older antipsychotic meds, with Haldol being used the most. These older ones can have some fairly strong and predictable side effects when used, especilly if used on the long term. Because of the potential and actual side effects, patient compliance becomes effected. But, they do work and have their place when needed. Drug companies have realized the need for newer meds with less side effects attached with them and have been creating newer antipsychotics....especially attempting to reduce the risk for extrapyramidal systems or EPS. EPS can range from akathisia (subjective anxiety and objective fidgetiness and agitation), dystonias (sustained uncomfortable/painful muscle contractions), pseudoparkinsonism (shuffling gait, stiffening of the muscles, drooling, and very blunted facial expressions), and tardive dyskinesia (possibly irreversible abnormal involuntary muscle movements).
Let me take for example, dystonias, which can often occur in the first 4 days of an antipsychotic's use. Cogentin is often used in conjunction with the antipsychotic med to prevent this from occuring. The thought behind using Cogentin is because it is a powerful anticholinergic drug. And why is this important? There is a balance between two types of neurotransmitters in the body....dopamine and acetylcholine. Antipsychotic meds artificially reduce dopamine levels which automatically shifts the level of acetylcholine as being higher in the body. When this occurs, an EPS can result. So, providing an anticholinergic drug to pull down acetylcholine is used to help provide a more equal balance between these two neurotransmitters...dopamine and acetylcholine. As a result, you can see Cogentin (a strong anticholinergic) used in two ways. One way, as a prophylactic...started at the same time as the antipsychotic...especially with these older antipsychotics...to prevent an EPS like dystonia from happening. Or in a second way, starting the antipsychotic by itself and only giving Cogentin IF an EPS occurs, like a dystonia. Some of the older antipsychotics have a higher potential for EPS than others. And some of the newer ones may present with lower or different risks for EPS. I hope this answered some of the question as to why Cogentin is used. It is not used to potentiate an antipsychotic's efficacy as an antipsychotic medication. Cogentin is used to reduce the risk of and to actively intervene upon this fairly predictable course of reactions....in this case, an EPS like dystonia or otherwise known as a dystonic reaction.
Now many, many psychiatric meds, if not all, may have the predictable side effect of sedation or drowsiness. Some meds are more sedating than others. For example, Mellaril is more sedating than Haldol. Desyrel/Trazodone (an antidepressant) is used as a sleeping pill. Many drugs have sedation. Cogentin can have a sedating effect as well...because anticholinergic meds are sedating in and of themselves. Some times, a doctor may use this sedation to an advantage at times when a patient is highly agitated and/or experiences extreme insomnia...and therefore needs to sleep or to become less agitated. The side effect of sedation may be, in this case, therapeutic. But again, many of these drugs can and are sedating to a lesser or greater amount depending upon the drug and dosage. Sedation as a side effect usually lessens over time. And if sedation is very troublesome, the doctor may reduce the amount of med, titrate up the dose differently, or switch to a less sedating medication. I hope this was helpful in relation to answering your question regarding sedation.