Topamax is an antiepileptic drug, prescribed to control both the mild attacks known as partial seizures and the severe tonic-clonic convulsions known as grand mal seizures. It is typically added to the treatment regimen when other drugs fail to fully control a patients attacks.
Topamax is also prescribed for the prevention of migraine headaches (also known as prophylactic treatment). However, due to a lack of studies, its not known whether the drug can treat acute migraine attacks.
Because Topamax sometimes causes confusion, dizziness, fatigue, and problems with coordination and concentration, you should not drive, operate machinery, or participate in any hazardous activity that requires full mental alertness until you are certain how the drug affects you.
Topamax has been known to cause a potentially serious condition known as metabolic acidosis (an increase of acid in the blood). In children, chronic metabolic acidosis may affect growth or cause rickets (a softening or weakness of the bones that can lead to bone deformities). Contact your doctor immediately if you experience symptoms of metabolic acidosis such as rapid breathing, an irregular heartbeat, confusion, lethargy, fatigue, or loss of appetite. Your doctor will decide if you should discontinue taking Topamax. Do not abruptly stop taking Topamax on your own; your doctor will gradually taper the dosage to avoid an increase in seizures.
Topamax has been known to trigger severe nearsightedness along with increased pressure inside the eye. The problem usually occurs within 1 month of starting treatment. If you develop blurred vision or eye pain, call your doctor immediately. Discontinuation of the drug may be necessary to prevent permanent vision loss.
In children with chronic diarrhea or untreated kidney disorders, use of Topamax may lead to rickets and reduced growth rates.
Tell your doctor if you have kidney problems or if you are on hemodialysis; your dosage of Topamax may need adjustment. Elderly patients in particular may experience reduced kidney function when taking Topamax. Also make sure the doctor is aware of any liver disorder you may have. Topamax must be used cautiously by individuals with impaired liver function.
Anyone using Topamax, particularly children, should be carefully monitored by their doctor for signs of increased body temperature or decreased sweating, especially during hot weather.
In rare instances, suicide attempts have been reported in people taking Topamax.
In vitro studies indicate that topiramate does not inhibit enzyme activity for CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2C19, CYP2D6, CYP2E1 and CYP3A4/5 isozymes.
Potential interactions between topiramate and standard AEDs were assessed in controlled clinical pharmacokinetic studies in patients with epilepsy. The effects of these interactions on mean plasma AUCs are summarized in Table 3.
In Table 3, the second column (AED concentration) describes what happens to the concentration of the AED listed in the first column when topiramate is added.
The third column (topiramate concentration) describes how the coadministration of a drug listed in the first column modifies the concentration of topiramate in experimental settings when Topiramate was given alone.
Table 3: Summary of AED Interactions with Topiramate
AED Co-administered AED Concentration Topiramate Concentration
Phenytoin NC or 25% increasea 48% decrease
Carbamazepine (CBZ) NC 40% decrease
CBZ epoxide NC NE
Valproic acid 11% decrease 14% decrease
Phenobarbital NC NE
Primidone NC NE
Lamotrigine NC at TPM doses up to 400 mg/day 15% increase
NC = Less than 10% change in plasma concentration.
AED = Antiepileptic drug.
NE = Not Evaluated.
TPM = Topiramate
In addition to the pharmacokinetic interaction described in the above table, concomitant administration of valproic acid and topiramate has been associated with hyperammonemia with and without encephalopathy.
Other Drug Interactions
Digoxin: In a single-dose study, serum digoxin AUC was decreased by 12% with concomitant Topiramate administration. The clinical relevance of this observation has not been established.
CNS Depressants: Concomitant administration of Topiramate and alcohol or other CNS depressant drugs has not been evaluated in clinical studies. Because of the potential of topiramate to cause CNS depression, as well as other cognitive and/or neuropsychiatric adverse events, topiramate should be used with extreme caution if used in combination with alcohol and other CNS depressants.
Oral Contraceptives: In a pharmacokinetic interaction study in healthy volunteers with a concomitantly administered combination oral contraceptive product containing 1 mg norethindrone (NET) plus 35 mcg ethinyl estradiol (EE), Topiramate given in the absence of other medications at doses of 50 to 200 mg/day was not associated with statistically significant changes in mean exposure (AUC) to either component of the oral contraceptive. In another study, exposure to EE was statistically significantly decreased at doses of 200, 400, and 800 mg/day (18%, 21%, and 30%, respectively) when given as adjunctive therapy in patients taking valproic acid. In both studies, Topiramate (50 mg/day to 800 mg/day) did not significantly affect exposure to NET. Although there was a dose dependent decrease in EE exposure for doses between 200-800 mg/day, there was no significant dose dependent change in EE exposure for doses of 50-200 mg/day. The clinical significance of the changes observed is not known. The possibility of decreased contraceptive efficacy and increased breakthrough bleeding should be considered in patients taking combination oral contraceptive products with Topiramate. Patients taking estrogen containing contraceptives should be asked to report any change in their bleeding patterns. Contraceptive efficacy can be decreased even in the absence of breakthrough bleeding.
Metformin: A drug-drug interaction study conducted in healthy volunteers evaluated the steady-state pharmacokinetics of metformin and topiramate in plasma when metformin was given alone and when metformin and topiramate were given simultaneously. The results of this study indicated that metformin mean Cmax and mean AUC0-12h increased by 18% and 25%, respectively, while mean CL/F decreased 20% when metformin was co-administered with topiramate. Topiramate did not affect metformin tmax. The clinical significance of the effect of topiramate on metformin pharmacokinetics is unclear. Oral plasma clearance of topiramate appears to be reduced when administered with metformin. The extent of change in the clearance is unknown. The clinical significance of the effect of metformin on topiramate pharmacokinetics is unclear. When Topiramate is added or withdrawn in patients on metformin therapy, careful attention should be given to the routine monitoring for adequate control of their diabetic disease state.
Lithium: Multiple dosing of topiramate 100 mg every 12 hrs decreased the AUC and Cmax of Lithium (300 mg every 8 hrs) by 20% (N=12, 6 M; 6 F).
Haloperidol: The pharmacokinetics of a single dose of haloperidol (5 mg) were not affected following multiple dosing of topiramate (100 mg every 12 hr) in 13 healthy adults (6 M, 7 F).
Amitriptyline: There was a 12% increase in AUC and Cmax for amitriptyline (25 mg per day) in 18 normal subjects (9 male; 9 female) receiving 200 mg/day of topiramate. Some subjects may experience a large increase in amitriptyline concentration in the presence of topiramate and any adjustments in amitriptyline dose should be made according to the patients clinical response and not on the basis of plasma levels.
Sumatriptan: Multiple dosing of topiramate (100 mg every 12 hr) in 24 healthy volunteers (14 M, 10 F) did not affect the pharmacokinetics of single dose sumatriptan either orally (100 mg) or subcutaneously (6 mg).
Risperidone: There was a 25% decrease in exposure to risperidone (2 mg single dose) in 12 healthy volunteers (6 M, 6 F) receiving 200 mg/day of topiramate. Therefore, patients receiving risperidone in combination with topiramate should be closely monitored for clinical response.
Propranolol: Multiple dosing of topiramate (200 mg/day) in 34 healthy volunteers (17 M, 17 F) did not affect the pharmacokinetics of propranolol following daily 160 mg doses. Propranolol doses of 160 mg/day in 39 volunteers (27M, 12F) had no affect on the exposure to topiramate at a dose of 200 mg/day of topiramate.
Dihydroergotamine: Multiple dosing of topiramate (200 mg/day) in 24 healthy volunteers (12 M, 12 F) did not affect the pharmacokinetics of a 1 mg subcutaneous dose of dihydroergotamine. Similarly, a 1 mg subcutaneous dose of dihydroergotamine did not affect the pharmacokinetics of a 200 mg/day dose of topiramate in the same study.
Others: Concomitant use of Topiramate, a carbonic anhydrase inhibitor, with other carbonic anhydrase inhibitors, e. g., acetazolamide or dichlorphenamide, may create a physiological environment that increases the risk of renal stone formation, and should therefore be avoided.
Drug/Laboratory Tests Interactions: There are no known interactions of topiramate with commonly used laboratory tests.
TOPAMAX® is contraindicated in patients with a history of hypersensitivity to any component of this product.