Definition
Central nervous system (CNS) stimulants are drugs that increase activity in certain areas of the brain. These drugs are used to improve wakefulness in patients that have narcolepsy. CNS stimulants are also used to treat patients that have attention deficit hyperactivity disorder (ADHD). There are four different types of central nervous system stimulants available in the United States: mixed amphetamine salts (brand name Adderall); dextroamphetamine (Dexedrine and Dextrostat); methylphenidate (Ritalin, Metadate, Methylin, and Concerta); and pemoline (Cylert).
Purpose
Central nervous system stimulants are used to keep patients who suffer from narcolepsy from falling asleep. Narcolepsy is a disorder that causes people to fall asleep during daytime hours.
These drugs are also used to treat behavioral symptoms associated with attention deficit hyperactivity disorder. Although it seems contradictory to give patients with ADHD drugs that are stimulants, these medications are often effective at treating symptoms of impulsivity, inattention, and hyperactivity, which are hallmark features of the disorder.
Description
The exact way that CNS stimulants work in treating narcolepsy and ADHD is not understood. The drugs' mechanism of action appears to involve enhanced activity of two neurotransmitters in the brain, norepinephrine and dopamine. Neurotransmitters are naturally occurring chemicals that regulate transmission of nerve impulses from one cell to another. A proper balance between the various neurotransmitters in the brain is necessary for healthy mental well-being.
Central nervous system stimulants increase the activities of norepinephrine and dopamine in two different ways. First, the CNS stimulants increase the release of norepinephrine and dopamine from brain cells. Second, the CNS stimulants may also inhibit the mechanisms that normally terminate the actions of these neurotransmitters. As a result of the dual activities of central nervous system stimulants, norepinephrine and dopamine have enhanced effects in various regions of the brain. Some of these brain areas are involved with controlling wakefulness and others are involved with controlling motor activities. It is believed that CNS stimulants restore a proper balance of neurotransmitters, which alleviates symptoms and features associated with narcolepsy and ADHD.
Although the intended actions of central nervous system stimulants are in the brain, their actions may also affect norepinephrine in other parts of the body. This can cause unwanted side effects such as increased blood pressure and heart arrhythmias due to reactions of norepinephrine on the cardiovascular system.
Recommended dosage
The usual dosage of amphetamine salts is 5–60 mg per day taken two or three times a day, with at least 4–6 hours between doses. The extended release form of amphetamine salts is taken as 10–30 mg once a day. Like amphetamine salts, the dose of immediate-release methylphenidate tablets is also 5–60 mg per day taken two or three times a day. Additionally, methylphenidate is available in sustained-release dosage forms and extended-release dosage forms, which are typically taken only once a day.
The usual dosage of dextroamphetamine is 5–60 mg per day given two or three times a day, with at least 4–6 hours between doses. A sustained-release form of dextroamphetamine is also available, which may be given once a day. The recommended dose of pemoline is 37.5–112.5 mg per day taken only once a day. However, due to pemoline's association with life-threatening liver dysfunction, pemoline is rarely used at the present time.
The therapeutic effects of central nervous system stimulants are usually apparent within the first 24 hours of taking the drugs. If effects are not evident, the dosages of CNS stimulants may be slowly increased at weekly intervals. CNS stimulants should always be used at the lowest effective dosages to minimize unwanted side effects. When the drugs are used for treating ADHD in children, therapy should be interrupted occasionally to determine whether symptoms reoccur and whether the drug is still necessary.
Precautions
Central nervous system stimulants are widely abused street drugs. Abuse of these drugs may cause extreme psychological dependence. As a result, new hand-written prescriptions must be obtained from physicians each month and any time a dosage adjustment is made. These drugs are best avoided in patients with a prior history of drug abuse.
CNS stimulants may cause anorexia and weight loss. Additionally, these drugs slow growth rates in children. Height and weight should be checked every three months in children who need to use these medications on a long-term basis.
The use of CNS stimulants should be avoided in patients with even mild cases of high blood pressure since the drugs may elevate blood pressure further.
Side effects
Central nervous system stimulants may increase heart rates and cause irregular heart rhythms, especially at high doses.
Symptoms of excessive stimulation of the central nervous system include restlessness, difficulty sleeping, tremor, headaches, and even psychotic episodes.
Loss of appetite and weight loss may also occur with central nervous system stimulants. It is necessary to monitor liver function regularly in patients who take pemoline since this drug has been associated with life-threatening liver disease.
Interactions
CNS stimulants should not be administered with certain types of antidepressant medications, including monoamine oxidase inhibitors (MAOIs) and selective serotonin reuptake inhibitors (SSRIs). Patients taking CNS stimulants should avoid MAOIs since the combination may elevate blood pressure to dangerously high levels, while SSRIs are best avoided since they may increase the central nervous system effects of CNS stimulants if the drugs are taken together.
Antacids may prevent CNS stimulants from being eliminated by the body and can increase the side effects associated with use of the stimulants.
Resources
BOOKS
Dipiro, J. T., R. L. Talbert, G. C. Yee, et al., eds. Pharmacotherapy: A Pathophysiologic Approach, 4th edition. Stamford, CT: Appleton and Lange, 1999.
Facts and Comparisons Staff. Drug Facts and Comparisons, 6th edition. St. Louis, MO: A Wolter Kluwer Company, 2002.
Kelly Karpa, PhD, RPh
