Basic Information
ID DDInter1821 and DDInter1312
Interaction Coadministration with oral contraceptives may significantly increase the plasma concentrations and pharmacologic effects of tizanidine. The proposed mechanism is inhibition of tizanidine metabolism via CYP450 1A2. The interaction has also been reported with other CYP450 1A2 inhibitors such as fluvoxamine, ciprofloxacin, and rofecoxib.
Management The use of tizanidine in combination with oral contraceptives or other CYP450 1A2 inhibitors should generally be avoided. Caution is advised if concurrent use is clinically necessary. Dosage adjustments may be required in patients who experience excessive adverse effects of tizanidine such as drowsiness, dizziness, lightheadedness, hypotension, bradycardia, or syncope.
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Alternative for Tizanidine M03B
Methocarbamol Chlormezanone Orphenadrine Cyclobenzaprine Baclofen Chlorzoxazone
Alternative for Norethisterone H01C
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Octreotide

G03A
Estetrol Etonogestrel Norelgestromin Medroxyprogesterone acetate
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Potential Metabolism Interactions
Substrate-Substrate Interaction：If more than one drug is metabolized by the same CYP, it is possible that its metabolism is inhibited because of the competition between the drugs. That means, it can be useful to lower the dosage of the drugs in the drug-cocktail because they remain longer in the organism than in monotherapy.
Inhibitor-Inhibitor Interaction：Combining two or more inhibitors of one CYP, should be compensated by lowering the dosage of these drugs because the metabolism is reduced and the drugs remain longer in the organism than in monotherapy. Not adapting the dosage bears the risk of even more side effects.
Inhibitor-Substrate Interaction：Combining drugs that have inhibitory effect and are substrates of one particular CYP, should be compensated by lowering the dosage. They rest longer in the organism than in monotherapy. Not adapting the dosage bears the risk of even more side effects.