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Drug Caution Code - Code D

Code D: Drowsiness

May cause drowsiness. Consult your pharmacist if you intend to drive a motorized vehicle or operate hazardous machinery. Avoid alcohol and drugs with sedative and/or relaxing effects.

This code refers to the sedating antihistamines and muscle relaxants:

First Generation Antihistamines

Potential for Drowsiness

Lower incidence of drowsiness overall.



Diphenhydramine and Doxylamine cause the most drowsiness and also exert significant anticholinergic activity



Drowsiness in some people, also causes dizziness



Significant anticholinergic activity







Muscle Relaxants



Antihistamines are most beneficial in the management of nasal allergies. They provide symptomatic relief of rhinnorhea, sneezing, oropharyngeal irritation or itching, lacrimation and red, irritated or itching eyes. Although there is a long history of use of antihistamines in treating a cold, their effectiveness is unclear. Histamine release is well documented in allergies and probably also occurs in viral infections as well. Chlorpheniramine has been shown to decrease the rhinorrhea and sneezing of a cold but this may be due to a drying effect on the mucosa. It works better early in a cold when a runny nose is present. Later, when nasal congestion and obstruction become more prominent, this drying effect provides little or no benefit and may even make the congestion worse.

SYMPTOMS OF SEDATION include: drowsiness, an inability to concentrate, dizziness, ataxia (loss of muscle coordination), and deep sleep. Sedation interferes with daytime activities and can alter the mind and slow reflexes so that accidents may occur. In some cases, sedation disappears after the antihistamine has been used for 2 or 3 days. Alcohol and other drugs that depress the central nervous system potentiate the sedative effect and should not be used concomitantly. The new non-sedating antihistamines are different enough to be classified separately and are not coded D. However, some patients may still experience sedation with these products.

ATROPINE-LIKE SIDE EFFECTS are also caused by classic antihistamines: dry mouth, blurred vision, tachycardia, central depression and stimulation, convulsant and anticonvulsant activity, antiemetic action, suppression of tremor, suppression of motion sickness. When combined with other drugs with atropine-like side effects (tricyclic antidepressants, anticholinergics), these side effects will be additive.

ANTIMUSCARINIC EFFECTS are also produced by antihistamines. They should be avoided or used with caution by patients with asthma (thickening of bronchial secretions), glaucoma (increased intraocular pressure), or prostate hypertrophy (urinary retention). The elderly are especially susceptible to side effects such as dry mouth and urinary retention.

Most antihistamines (especially promethazine) have mild local anesthetic properties and patients should be advised of this effect.

TOLERANCE or an apparent decrease in effectiveness may develop after a period of weeks or months of continuous use due to hepatic enzyme induction. A different antihistamine can be recommended to provide continued relief of symptoms. The original drug may be effective again after a period of non-use.

PROMETHAZINE should NOT be used for CHILDREN less than 2 years of age because of decreased metabolism and excretion, and because of a possible link with Sudden Infant Death Syndrome and sleep apnea in infants. It is excreted in breast milk, and should not be used by nursing rnothers. Promethazine is not recommended for children less than 6 years of age.

Antihistamines can cause a paradoxical reaction of hyperexcitability in pediatric and geriatric patients. Symptoms include: unusual excitement, nightmares, irritability, restlessness or euphoria. Dizziness,sedation, confusion, and hypotension are more likely to occur in geriatric patients.

Muscle Relaxants

Muscle relaxants do not directly relax skeletal muscle but work via a central effect. 1t is not clear whether the muscle relaxant properties are a specific effect or simply an extension of the central sedative effect. Their efficacy has not been rigorously established but they appear to benefit certain situations. Orphenadrine may have analgesic properties; it does have anticholinergic and some antihistamne activity.

Treatment Alternatives

Recommend a non-drowsy antihistamine (loratidine, fexofenadine, cetirizine, etc). If drowsiness is a problem with one of these, try a different one.
Recommend a preparation which does not contain an antihistamine. Decongestants, cough suppressants (except codeine) and expectorants do not cause drowsiness.
Recommend using an antihistamine containing product only at night, and a non-drowsy product during the day.
Instead of a muscle relaxant, recommend rest and physical therapy. Hot/cold packs could also be recommended. Sometimes, a simple pain reliever (acetaminophen) may be all that is needed.

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