For three decades, the mainstay of treatment for Parkinson's has been the drug levodopa. Nerve cells in the brain convert levodopa into dopamine, which relieves some of the tremors and stiffness -- the most common and troubling symptoms of Parkinson's. Because enzymes in the blood tend to break down levodopa before it can reach the brain, it's typically combined with the drug carbidopa, which keeps levodopa intact until it gets to the brain. Although levodopa is effective, it can cause side effects ranging from dry mouth to nausea and vomiting.
Drugs called dopamine agonists mimic the effects of dopamine in the brain. They stimulate the same parts of the brain that normally receive dopamine, so that the brain thinks it's getting dopamine. Dopamine agonists such as bromocriptine (brand name Parlodel), pramipexole (brand name Mirapex) and ropinirole (brand name Requip) can be used instead of levodopa or along with it, depending on the person's symptoms. In some people these drugs can increase the risk of compulsive behaviors, including gambling and sex.
Antivirals are normally used to treat diseases caused by a virus, but doctors have discovered that they can also relieve the symptoms of Parkinson's disease. Amantadine (brand name Symmetrel) is an antiviral drug that reduces tremor, slow movements and fatigue in Parkinson's patients. It can be given alone, or combined with levodopa or a dopamine agonist.
The first drugs used to treat Parkinson's were anticholinergic medications. They work by decreasing the activity of the chemical messenger acetylcholine which also controls movement. Anticholinergic medications can be taken alone or with levodopa and may help with tremors and rigidity in people with mild Parkinson's disease. However, anticholinergic drugs are rarely used in the elderly because they can cause confusion.
Monoamine oxidase-B (MAO-B) inhibitors like selegiline (brand name Eldepryl) and rasagiline (brand-name Azilect) inhibit an enzyme called monoamine oxidase B (MAO-B), which normally breaks down dopamine in the brain. By blocking the effect of this enzyme, MAO-B inhibitors enable both natural dopamine and levodopa to work longer in the brain. One downside to these drugs is that they can interact with other medications, including antidepressants, narcotic painkillers and decongestants.
The class of drugs known as catechol O-methyltransferase (COMT) inhibitors also blocks an enzyme that normally breaks down dopamine. The COMT inhibitor entacapone is often added to levodopa and carbidopa (the combination drug is called Stalevo) to help levodopa stay active in the brain for a longer period of time.
Although these drugs can help with symptoms, people who use them for long periods of time may find that the benefits come and go. They may have periods in which they can move smoothly, followed by periods in which they have trouble moving. This phenomenon is called "wearing off." Another side effect of using Parkinson's medications over the long-term is abnormal movements (dyskinesia). Eventually, the effects of medication become so limited that some people turn to surgery to relieve their symptoms. We'll find out about that, next.