What is Drug Interaction?Medicines are used to treat certain conditions or illnesses, but when two or more medicines are used simultaneously, they may interact with each other and cause complications or unwanted side effects. Drug interactions may occur with prescription drugs, over-the-counter (OTC) medicines, herbal remedies, or dietary supplements, so it is important that the patient realizes the dangers of mixing drugs without consulting a doctor.Parkinson’s Disease MedicinesThe drug therapy most commonly used in Parkinson’s disease usually consists of the following : LevodopaLevodopa + peripheral decarboxylase inhibitors such as carbidopa and benserazide.Dopamine receptor agonists – pramipexole, ropinirole, and rotigotine.MAO-B inhibitors – selegiline and rasagiline.COMT inhibitors – entacaponeAnticholinergics – benzhexol and orphenadrineAmantadine.Drug Interactions with Parkinson’s Disease MedicinesLevodopaSince this is the most widely used medicine in Parkinson’s disease, its drug interactions should be understood by all Parkinson’s patients. Certain medicines decrease effectiveness of levodopa hence the dose of levodopa may have to be adjusted if used together. These include Anticholinergics – sometimes used in addition to levodopa for treatment of symptoms in Parkinson’s disease. Over-the-counter cough and cold medicines contain anticholinergics such as diphenhydramine, chlorpheniramine, and phenylephrine. Antispasmodics used for treating stomach or uterine cramps also contain anticholinergics such as dicyclomine and hyoscyamine. Anti-allergic medicines (like