Dopamine-Replacement Medications

Dopamine-replacement medications help relieve the manifestations of Parkinson’s disease and when combined with carbidopa to make it possible to reduce the dosage of levodopa and prevent some of its adverse effects. Levodopa is no longer available as a single medication – it is always combined with carbidopa or carbidopa and entacapone.

Prototype and Other Medications

Medication Classification – Dopamine-Replacement Drugs

The prototype medication for dopamine-replacement medications is levodopa/carbidopa. This combination medication contains both levodopa and carbidopa. Carbidopa augments levodopa by decreasing the amount of levodopa that converts to dopamine in the intestine and periphery. This results in greater amounts of levodopa reaching the CNS.

Expected Pharmacologic Action

Dopamine-replacement medications work because they can cross the blood-brain barrier, where they are then taken up by the remaining dopaminergic neurons in the substantia nigra. The medication then converts to dopamine in these neurons and is available for use.

Adverse Drug Reactions

Adverse effects of dopamine-replacement medications include nausea, vomiting, and orthostatic or postural hypotension. Dopamine-replacement medications may also darken sweat and urine. Dyskinesias, which are movement disorders, are also common during the initial phase of treatment. This seems counterproductive because providers use dopamine-replacement medications to reduce the tremors and rigidity of Parkinson’s disease. However, these effects can occur and may be related to inappropriate dosing or medication schedules.

Interventions

For some clients, a decrease in dosage may be enough to decrease the incidence of dyskinesias. If not, talk to the provider about prescribing amantadine, also called Symmetrel, to decrease dyskinesias. Dopamine-replacement medications can cause orthostatic hypotension, which is sometimes called postural hypotension, putting clients at risk for falls.

Safety Alert

Closely monitor clients for orthostatic or postural hypotension and institute measures to prevent falls, such as having them sit on the side of the bed before standing and assisting with ambulation. It is also important to measure the blood pressure of clients taking these medications when they are lying, sitting, and standing, with a 1- to 3-minute break between each reading. A decrease in systolic blood pressure of 20 mm Hg or a decrease of 10 mm Hg of diastolic pressure indicates orthostatic hypotension.

Administration

When administering a dopamine-replacement medication, begin with a low dose and gradually increase the dose to reduce the adverse effects. Advise clients that it may take up to 6 months to achieve the full therapeutic response. Immediate-release tablets begin working within 30 minutes, but then begin to wear off. Extended-release tablets work over 4 to 6 hours but can take up to 2 hours to begin working in the morning. In relation to dosing schedules, monitor for any loss of medication effect and “on/off” episodes that indicate periods when too little dopamine is in the bloodstream.

Safety Alert

Regulate the dose of dopamine-replacement medications so serum levels are adequate for consistent control of manifestations. Times of the day when muscle rigidity and tremors recur is called the “on-off” phenomenon. Clients who have these episodes need regulation of both the time you administer dopamine-replacement medications and possibly the dosage. This ensures safe ambulation and engagement in daily activities, without periods of neuromuscular compromise.

Client Instructions

Advise clients to take dopamine-replacement medications with food if necessary. Make sure they avoid high-protein foods because they decrease the absorption of levodopa/carbidopa. Warn clients that their urine and sweat may darken, but this is not a side effect that should cause concern. Discuss the potential for orthostatic hypotension with clients, along with instructions to move slowly to a sitting or standing position, and to ask for help during ambulation if they feel dizzy.

Safety Alert

Be sure to include caregivers when giving instructions about how to take a dopamine-replacement medications and adverse effects that indicate the need for an adjustment in dosage. Clients who experience manifestations of psychosis or dyskinesias may not recognize that this is occurring. Thus, the people who spend time with them should know what to report in regard to hallucinations and paranoid delusions.

Contraindications and Precautions

Contraindications for dopamine-replacement medications include clients who have angle-closure glaucoma, a history of melanoma, psychosis, or suicidal thoughts. Use dopamine-replacement medications with caution for older adults or clients with existing renal, hepatic, respiratory, or endocrine disorders, wide-angle glaucoma, peptic ulcer disease, or depression or bipolar disorder.

Interactions

Traditional first-generation antipsychotics, such as haloperidol and chlorpromazine, and supplements that contain peroxidine (vitamin B6) decrease the action of levodopa/carbidopa. High-protein meals delay the absorption of levodopa/carbidopa, making its effects unpredictable and erratic. Anticholinergic medications increase a client’s response to levodopa/carbidopa because they alter the balance between dopamine and acetylcholine in the brain by blocking the release of acetylcholine. Another important interaction can be the timing of taking levodopa/carbidopa with a monoamine oxidase inhibitor antidepressant, or MAOI.

Safety Alert

Be sure to ask clients with a new prescription for levodopa/carbidopa if they took an MAOI antidepressant in the past 2 weeks. A hypertensive crisis can occur if clients take levodopa/carbidopa and MAOIs within 2 weeks of each other.