Antispasmodic Agents

Cyclobenzaprine (Flexeril®, Ortho-McNeil-Janssen, New Brunswick, NJ), 5 mg tid; may increase to 10 mg tid. Anticholinergic effects (drowsiness, urinary retention, dry mouth): avoid in elderly; QT prolongation: avoid in patients with arrhythmias, cardiac conduction disturbances, heart block, heart failure, or recent myocardial infarction; may raise intraocular pressure: avoid in patients with glaucoma; elimination half-life ~18 h in young subjects, ~33 h in elderly, and ~46 h in patients with hepatic impairment.

Carisoprodol (Soma®R , Wallace Laboratories, Abbot Park, IL) 350 mg qid, is not recommended in children under 12 years of age; drowsiness; can cause psychological and physical dependence, withdrawal symptoms can occur with discontinuation; excessive use, overdose, or withdrawal may precipitate seizures; reports describe idiosyncratic or allergy-type reactions after first dose (mental status changes, transient quadriplegia, fever, angioneurotic edema, asthmatic episodes) metabolized to meprobamate, a barbiturate and so there is a significant risk of dependency. Rapid cessation can lead to delayed seizure when patients are taking higher doses.

In this author's experience, a patient who was taking six Soma®R per day was admitted to the hospital for surgery and her Soma medication was not restarted after surgery. On her 5th postoperative day, she had a seizure which was originally attributed to the antibiotics. It is easy to lose track of long-term medications when patients are hospitalized.

Chlorzoxazone (Paraflex®, Ortho-McNeil-Janssen, New Brunswick, NJ) 250-750 mgtid or qid, causes dizziness and drowsiness, rare cases of hepatotoxicity, gastrointestinal irritation, and rare cases of gastrointestinal bleeding; may cause red or orange urine; avoid in patients with liver impairment.

Metaxalone (Skelaxin®R , Myung Moon Pharm, Korea) 800 mg tid-qid. Not recommended in children <12 years; do not use in patients with renal or hepatic failure or a history of anemia; dizziness and drowsiness may occur, and in rare cases leukopenia or hemolytic anemia may result.

Methocarbamol (Robaxin®, Schwarz Pharma, Mequon, WI) 1,500 mg qid for 72 h then 1,000 mg qid. Available as an injectable but should not be injected in patients with renal failure; may cause brown-to-black or green discoloration of urine; may impair mental status; may exacerbate symptoms of myasthenia gravis.

Orphenadrine (Norflex®, 3 M Pharmaceuticals, St. Paul, MN) 100 mg bid has anticholinergic effects such as drowsiness, urinary retention, and dry mouth. This drug should be avoided in the elderly; it may raise intraocular pressure and therefore must be avoided in patients with glaucoma. Orphenadrine is associated with gastrointestinal disturbances. Its elimination half-life is 13-20 h, and elimination is extended when use is prolonged. Finally, orphenadrine should be avoided in patients with cardiospasm or myasthenia gravis, and is generally contraindicated in duodenal or pyloric obstruction or stenosing peptic ulcers.

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