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Effects of Topiramate on other anti-epileptic medicines: If Topiramate is taken concomitantly with other anti-epileptic medicines (phenytoin, carbamazepine, valproic acid, phenobarbital, primidone), there is no effect on their steady-state plasma concentrations, except in very rare cases where the addition of Topiramate to phenytoin may result in an increase of plasma concentrations of phenytoin. Consequently, any patient on phenytoin, taking Topiramate, in addition, should have phenytoin levels monitored.
Effects of other anti-epileptic medicines on Topiramate: Phenytoin and carbamazepine decrease the plasma concentration of Topiramate. The addition or withdrawal of phenytoin or carbamazepine to Topiramate therapy may require an adjustment in dosage of Topiramate. This should be done by titrating to clinical effect.
The withdrawal or addition of valproic acid does not produce clinically significant changes in plasma concentrations of Topiramate and, therefore, dosage adjustment of Topiramate is not necessary.
Digoxin: Serum digoxin levels have decreased with concomitant administration of Topiramate. When Topiramate is added or withdrawn in patients on digoxin therapy, careful attention should be given to the routine monitoring of serum digoxin.
Oral Contraceptives: Efficacy of oral contraceptives may be compromised when used concurrently with Topiramate. Bearing in mind the potential risk of teratogenicity, patients should receive a preparation containing not less than 50 micrograms of oestrogen or use some alternative non-hormonal method of contraception as Topiramate increases plasma clearance of the oestrogenic component significantly. Patients on oral contraceptives are advised to report any change in their bleeding patterns.
Others: Concomitant use of Topiramate with agents predisposing to nephrolithiasis (renal stone formation) should be avoided.
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