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Flexeril

Cyclobenzaprine is a skeletal muscle relaxant and a Central Nervous System (CNS) Depressant. It is marketed as Flexeril (5 and 10 mg tablets). The 10 milligram tablets are available generically. more...

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Mechanism of Action

The exact mechanism of action for cyclobenzaprine is unknown. Current research appears to indicate that cyclobenzaprine acts on the locus coeruleus where it results in increased norepinephrine release, potentially through the gamma fibers which innervate and inhibit the alpha motor neurons in the vetral horn of the spinal cord. Decreased firing of the alpha motor neuron results in decreased muscular tone.

Indications

Cyclobenzaprine is typically prescribed to relieve pain and muscle spasms. Typically, muscle spasms occur in an injury to stabilize the affected body part and prevent further damage. The spasm of the muscles can actually increase the pain level. It is believed that by decreasing muscular spasm, pain is diminished. A common application would be that of a whiplash injury in a car accident.

It is also prescribed off-label as a sleep-aid.

Side Effects

Common side effects include drowsiness, dizziness, and blurred vision. Other side effects are respiratory depression and decreased functionality in various muscles.

Legality

Cyclobenzaprine is regulated in the U.S. for prescription only. Cyclobenzaprine is unscheduled, however, and it is not illegal to have cyclobenzaprine in your possesion, even without a prescription.

Abuse

Cyclobenzaprine is not widely abused, despite having an arguably high potential for abuse. As a generality, habitual drug users tend to steer clear of anti-depressants, because of the possibility of contraindications with other psychoactive drugs. Cyclobenzaprine, on the other hand, can induce moderate to severe anticholinergic effects at higher doses, as well as benzodiazepine-like sedation and often pleasurable muscle-relaxation. At even higher doses, cyclobenzaprine may cause severe ataxia, and due to excessive muscle-relaxation, and possibly disorienting side-effects such as a floating sensation or other imagined movements (usually experienced when at rest.) Side-effects such as these are directly related to the favoritism of newer, more mild antidepressant medications over tricyclic antidepressants. Although purportedly unpleasant, cyclobenzaprine is relatively benign in case of overdose, depending on it's toxicity level in the user, and also on the susceptibility of the user to possibly harmful effects of overdose. Note that the susceptibility to these potentially damaging effects are greatly increased when cyclobenzaprine is used in conjunction with other drugs, particularly Central Nervous System Depressants and other antidepressants. Use of cyclobenzaprine with a MAOI (Mono Amine Oxidase Inhibitor) will very possibly result in fatality. Use of cyclobenzaprine with an SSRI (Selective Seratonin Reuptake Inhibitor) is not recommended and could lead to unpleasant and possibly damaging interactions. No deaths have been associated with cyclobenzaprine overdose, and permanent damage is almost always related to overactivity of relaxed muscles or contraindications with other drugs.

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Calculations
From International Journal of Pharmaceutical Compounding, 1/1/05 by Prince, Shelly J

1 The usual adult dose of cyclobenzaprine (Flexeril) is 10 mg 3 times a day. What would be the dose for an 8-year-old girl?

If the only information available forthe child is her age, Young's Rule or Cowling's Rule can be used to calculate an estimated dose.1

Young's Rule: age/(age + 12) × adult dose = dose for child

8/(8 + 12) × 10 mg = 4 mg 3 times a day

Cowling's Rule: age at next birthday/24 × adult dose = dose for child

9/24 × 10 mg = 3.75 mg 3 times a day

Notice that, in both of these equations, the child's age is in years.

A. You receive more information about this child and learn that she weighs 47 lbs. How much Flexeril should she receive per dose?

When the child's weight is available, you can use dark's Rule1 to provide a more accurate dose than the dose calculated based on the child's age.

Clark's Rule: weight/150 × adult dose = dose for child

For this equation, the child's weight is in pounds; and 150 is assumed to be the average adult weight in pounds.

47 lbs/150 lbs × 10 mg = 3.13 mg 3 times a day

B. You measure the child and find that she is 3'1'' tall. How much Flexeril should she receive per dose?

Since you have the patient's height and weight, you can find her body surface area (BSA) using a nomogram.2 Based on this information, the patient's BSA would be 0.71 m^sup 2^.

The dose based on BSA can then be calculated using the following equation:1

BSA/1.73 m^sup 2^ × adult dose = dose for child

For this equation, the child's BSA is in square meters, and 1.73 m^sup 2^ is assumed to be the average adult BSA.

0.71 m^sup 2^/1.73 m^sup 2^ 10 mg = 4.104 mg 3 times a day

C. You decide to use a dose of 4 mg based on the dose calculated above and prepare a cyclobenzaprine suspension with a concentration of 4 mg/tsp. How long will 120 ml of this suspension last?

120 mL × 4 mg/5 mL × 1 dose/4 mg × 1 day/3 doses = 8 days

D. What would be the percent strength and ratio strength of the suspension?

4 mg/5 ml × 1 g/1000 mg × 100 = 0.08% w/v

5 mL/4 mg × 1000 mg/g = 1250 mL/g = 1:1250 w/v

2 You need to prepare 20 pediatric suppositories, each containing 80 mg of drug. You know that the calibrated volume of the suppository mold is 1.2 mL; therefore, you weigh 80 mg of the drug and mix it with 500 mg of the suppository base you will be using to prepare the prescription. You melt this mixture and pour it into a cavity in your suppository mold. You also melt a small amount of the suppository base alone and pour it into the cavity to completely fill it. After the suppository has cooled, you remove it from the mold and find that it weighs 1.35 g. How much drug and suppository base will you need to prepare this prescription? (allow for two extra suppositories)

1.35 g/supp - 0.08 g drug/supp = 1.27 g of base/supp

80 m g/supp × 22 su pp × 1 g/1000 mg = 1.76 g of drug needed

1.27 g/supp × 22 supp = 27.94 g of base needed

3 The pediatric dose for ciprofloxacin after exposure to anthrax is 15 mg/kg every 12 hours for 60 days, not to exceed a daily dose of 500 mg.3 What would be the daily dose and total dose of ciprofloxacin over the course of therapy for a patient weighing 35 lbs?

15 mg/kg/dose × 1 kg/2.2 lbs. × 35 lbs × 2 doses/day = 477.27 mg/day

477.27 mg/day × 60 days × 1 g/1000 mg = 28.64 g total

4 A mother mistakenly buys Tylenol infant's drops rather than the Tylenol children's elixir for her 18-month-old child. She calls the pharmacy to ask a bout the difference in strength because she usualIy buys the elixir and gives her child a dose of 3/4 teaspoonful. However, she did not notice that she had bought the drops and gave her child the "usual" dose of 3/4 teaspoonful. The concentration of acetaminophen in Tylenol children's elixir is 160 mg/5 mL, and the concentration of acetaminophen in Tylenol infant's drops is 100 mg/mL.3 How much additional acetaminophen did the child receive from the Tylenol infant's drops?

3/4 tsp × 5 mL/tsp × 160 mg/5 mL = 120 mg from the usual dose of elixir

3/4 tsp × 5 mL/tsp × 100 mg/mL = 375 mg from drops

375 mg - 120 mg = 255 mg additional acetaminophen received

References

1. Ansel HC, Stoklosa MJ. Pharmaceutical Calculations. 11th ed. Baltimore, MD: Lippincott Williams &Wilkins; 2001: 73,76.

2. Lentner C, ed. Geigy Scientific Tables. 8th ed. West Caldwell, NJ: CibaGeigy Corp.; 1981: 227.

3. Short RM, Snitker JA, eds. CliniSphere 2.0 [book on CD Rom]. St. Louis, MO: Facts and Comparisons; 2004.

Shelly J. Prince, PhD, RPh

Southwestern Oklahoma State University

College of Pharmacy

Weatherford, Oklahoma.

Address correspondence to: Shelly J. Prince, PhD, RPh, College of Pharmacy, Southwestern Oklahoma State University, 100 Campus Drive, Weatherford, OK 73096.

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