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Anticonvulsants are used to stop an ongoing seizure or to decrease the frequency or severity of anticipated future seizures. During a seizure episode or status epilepticus, the route of administration for anticonvulsants is IV (see Systemic Pharmacotherapeutics of the Nervous System: Drugs Used for Treatment of Status Epilepticus ). For longterm maintenance, the oral route is preferred, although absorption may be limited or variable depending on the drug used (see Systemic Pharmacotherapeutics of the Nervous System: Anticonvulsant Drugs). SC or IM injections are seldom used because of the variability in drug absorption.
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Table 1
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Drugs Used for Treatment of Status Epilepticus |
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Drug
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Species and Dosage
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Diazepam
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Dogs, cats 0.5–2.0 mg/kg, IV bolus; can be repeated 2–3 times at intervals of 5–10 min; CRIa 0.5–2.0 mg/kg/hr
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Foals: 0.05–0.4 mg/kg, IV slowly
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Adult horses: 25–50 mg/horse, IV
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Ruminants: 0.5–1.5 mg/kg, IV or IM
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Phenobarbital
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Dogs, cats: 2–4 mg/kg/ IV bolus; can be repeated at 20- to 30-min intervals until a total dosage of 20 mg/kg is reached; CRI 3–10 mg/hr to effect
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Foals, adult horses: loading dose of 12–20 mg/kg, IV over 20 min, then 6.65–9 mg/kg, IV over 20 min every 8–12hr
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Sodium pentobarbital
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Dogs, cats: 2–15 mg/kg, IV, to effect to stop motor activity
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Foals, adult horses: 2–4 mg/kg, IV to effect
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Propofol
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Dogs, cats: 2.5–4.0 mg/kg, IV, to effect to stop motor activity; CRI: 0.1–0.3 mg/kg/min to effect
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Phenytoin
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Dogs 2–5 mg/kg as slow IV infusion
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Foals, adult horses: 5–10 mg/kg, IV followed by 1–5 mg/kg, IV, IM, or PO every 2–4 hr until seizures stop and maintenance dose started
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a CRI: constant rate infusion
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Table 2
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Anticonvulsant Drugs |
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Anticonvulsant Drug
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Dosage and Frequency
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Half-life
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Time to Steady State
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Therapeutic Level
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Adverse Effects/Comments
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First-Line Maintenance Anticonvulsant Drugs
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Phenobarbital
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Adjust dosage in all species by monitoring serum levels and seizure diary.
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Dogs: 2–4 mg/kg, PO, bid (starting dose); up to 10 mg/kg, bid
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40–90 hr (Beagles 25–38 hr)
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10–24 days
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15–45 μg/mL (66–200 μmol/L), preferably keep values within 20–35 μg/mL (85–150 μmol/L)
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Sedation, polydipsia, induces P450 system, increase in liver enzymes; liver failure is uncommon.
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Cats: 2–4 mg/kg, PO, bid (starting dose)
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34–43 hr
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10–30 μg/mL
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Liver enzymes do not typically increase in cats.
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Horses: 3–5 mg/kg, PO, sid as a starting dose; up to 11 mg/kg, PO, sid
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18 hr
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10–40 μg/ml (43–175 μmol/L)
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Foals: As above
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13 hr
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Ruminants: 11 mg/kg, PO sid
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Bromide (potassium salt)
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Dogs, cats: 20–40 mg/kg, PO, sid or divided bid if GI upset.
Dogs: loading dose 400–600 mg/kg, PO divided into 4 doses, given over 1–4 days
Horses: 90 mg/kg/day, PO
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Dogs: 20–46 days
Cats: 10 days
Horses: 5 days
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Dogs: 100–200 days
Cats: 6 wk
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Bromide alone: 1–3 mg/mL (15–20 μmol/L)
Bromide/phenobarbital combined: 1–2 mg/mL
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Sedation, weakness, polydipsia, vomiting, polyphagia, skin rash. Reduce dose with renal insufficiency. High chloride intake increases bromide elimination. Chloride content of diet should be stable. Use with extreme caution in cats and monitor with thoracic radiographs because bronchial/asthmatic signs may be fatal in cats.
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Bromide (sodium salt)
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17–30 mg/kg, PO, sid or divided bid if GI upset. The dose of sodium bromide is less than that of potassium bromide to account for the higher bromide content.
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See potassium bromide notes.
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Diazepam
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Dogs: 0.5–2 mg/kg per rectum at onset of seizure; repeat up to 3 times in 24 hr
Cats: 0.25-0.5 mg/kg, PO, bid-tid
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Dogs: 2.5–3.2 hr
Cats: 5.5 hr
Horses: 7–22 hr
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While oral diazepam is not effective in dogs, rectal administration can be used to stop cluster seizures or status epilepticus. Oral diazepam can be used in cats as a maintenance drug; sedation, liver failure are potential problems in cats.
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Second-Line (Add-on) Anticonvulsant Drugs
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Clonazepam
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Dogs: 0.1–0.5 mg/kg, PO, bid-tid
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1.5–3 hr
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22–77 ng/mL
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Extremely potent benzodiazepine; sedation, withdrawal signs if drug stopped abruptly.
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Clorazepate
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Dogs: 2–6 mg/kg, PO, bid-tid
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5–6 hr
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1–2 days
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20–75 μg/L
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15 times less potent than clonazepam; sedation, withdrawal seizures.
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Felbamate
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Dogs: 15 mg/kg, PO, tid; increase by 15 mg/kg biweekly until seizures controlled; maximal (toxic) dosage 300 mg/kg
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5–6 hr
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1 day
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125–250 μmol/La
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Blood dyscrasia; induces P450 system, liver disease. Use with care with other potentially hepatotoxic drugs.
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Gabapentin
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Dogs: 10 mg/kg, PO, tid; up to 30–60 mg/kg, tid
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3–4 hr
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<24 hr
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4–16 mg/La (70–120 μmol/L)
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Sedation, dizziness, ataxia, fatigue, diarrhea; reduce dose with renal dysfunction.
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Levetiracetam
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Dogs: 20 mg/kg, PO, tid
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4–10 hr
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2–3 days
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35–120 μmol/La
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Restlessness, vomiting, ataxia at dosages >400 mg/kg/day.
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Topiramate
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Dogs: 5–10 mg/kg/day, PO, bid
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2–4 hr
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3–5 days
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2–25 mg/L (15–60 μmol/L)a
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GI upset, irritability
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Valproic acid
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Dogs: 10–60 mg/kg, PO, tid
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90–120 min
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<24 hr
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Probably ineffective due to very short half-life; liver toxicity and pancreatitis.
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Zonisamide
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Dogs: 4–8 mg/kg/day, PO, divided; up to 10 mg/kg, bida
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15–20 hr
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3–4 days
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10–40 mg/L (45–180 μmol/L)
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Sedation, ataxia, loss of appetite
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a Therapeutic range established for humans
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Last full review/revision March 2012 by Linda Shell, DVM, DACVIM-Neurology
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