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Contac 12 Hour Allergy Side Effects

Generic name: clemastine

Medically reviewed by Last updated on Jan 22, 2024.

Note: This document contains side effect information about clemastine. Some dosage forms listed on this page may not apply to the brand name Contac 12 Hour Allergy.

Applies to clemastine: oral oral solution, oral tablets.

Side effects include:

Sedation, sleepiness, dizziness, disturbed coordination, epigastric distress, thickening of bronchial secretions.

For Healthcare Professionals

Applies to clemastine: compounding powder, oral syrup, oral tablet.

Nervous system

Nervous system side effects have included central nervous system depression which has commonly resulted in drowsiness, dizziness, and headache. Dyskinesias have rarely been reported following chronic use of antihistamines.[Ref]

Nearly all patients experience drowsiness. This drowsiness may subside in some patients with extended use. Patients should be warned against driving, as well as concomitant ingestion with alcohol and other sedative-hypnotic drugs.

Few cases of dyskinesias and tremors, often of the face, have been reported in patients whose chronic use of antihistamines extended over a period of 3 to 10 years. Some of these cases were only partially relieved by discontinuation of the drug. Haloperidol was successful in relieving symptoms.[Ref]


Gastrointestinal side effects have included dry mouth and constipation due to anticholinergic effects. This has occurred in up to one-third of patients.[Ref]


Cardiovascular side effects have included hypotension, tachycardia, and palpitations.[Ref]


Ocular effects are due to the anticholinergic effects and may include blurred vision, diplopia, and dry eyes.[Ref]


Genitourinary side effects have included dysuria, urinary hesitancy, decrease in urine flow, and in rare cases, acute urinary retention.[Ref]


Hematologic side effects that have been reported with antihistamines include bone marrow suppression, thrombocytopenia, and aplastic anemia.[Ref]

A fatal case of agranulocytosis has been reported in a patient taking chlorpheniramine, pseudoephedrine, acetaminophen, dextromethorphan, phenylpropanolamine, and aspirin. Chlorpheniramine was felt to be the cause.[Ref]


Respiratory side effects have included thickening of bronchial secretions, wheezing, and tightness in the chest.[Ref]


Dermatologic side effects have included toxic pustuloderma.[Ref]

At least one case of toxic pustuloderma has been reported during clemastine therapy.[Ref]


1. Thomas JS, Heurich AE, Ralph JW, Crane R, Shepherd DA (1977) "Double-blind, controlled study of clemastine fumarate, chlorpheniramine and placebo in patients with seasonal allergic rhinitis." Ann Allergy, 38, p. 169-74

2. (1976) "Letter: Dyskinesia associated with chronic antihistamine use." N Engl J Med, 294, p. 113

3. Thach BT, Chase TN, Bosma JF (1975) "Oral facial dyskinesia associated with prolonged use of antihistaminic decongestants." N Engl J Med, 293, p. 486-7

4. Schuller DE, Turkewitz D (1986) "Adverse effects of antihistamines." Postgrad Med, 79, p. 75-86

5. Frolund L, Etholm B, Irander K, Johannessen TA, Odkvist L, Ohlander B, Weeke B (1990) "A multicentre study of loratadine, clemastine and placebo in patients with perennial allergic rhinitis." Allergy, 45, p. 254-61

6. (2001) "Product Information. Tavist (clemastine)." Sandoz Pharmaceuticals Corporation

7. Schran HF, Petryk L, Chang CT, Oconnor R, Gelbert MB (1996) "The pharmacokinetics and bioavailability of clemastine and phenylpropanolamine in single-component and combination formulations." J Clin Pharmacol, 36, p. 911-22

8. Turner RB, Sperber SJ, Sorrentino JV, OConnor RR, Rogers J, Batouli AR, Gwaltney JM (1997) "Effectiveness of clemastine fumarate for treatment of rhinorrhea and sneezing associated with the common cold." Clin Infect Dis, 25, p. 824-30

9. Gwaltney JM, Park J, Paul RA, Edelman DA, Oconnor RR, Turner RB (1996) "Randomized controlled trial of clemastine fumarate for treatment of experimental rhinovirus colds." Clin Infect Dis, 22, p. 656-62

10. Hardin AS (1988) "Chlorpheniramine and agranulocytosis ." Ann Intern Med, 108, p. 770

11. Eisner EV, LaBocki NL, Pinckney L (1975) "Chlorpheniramine-dependent thrombocytopenia." JAMA, 231, p. 735-6

12. Kanoh T, Jingami H, Uchino H (1977) "Aplastic anaemia after prolonged treatment with chlorpheniramine ." Lancet, 1, p. 546-7

13. Deringer PM, Maniatis A (1976) "Chlorpheniramine-induced bone-marrow suppression." Lancet, 1, p. 432

14. Feindkoopmans A, Vandervalk PGM, Steijlen PM, Vandekerkhof PCM (1996) "Toxic pustuloderma associated with clemastine therapy." Clin Exp Dermatol, 21, p. 293-5

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Some side effects may not be reported. You may report them to the FDA.