UK Clinical Pharmacy Association


Issues for surgery

Precipitation of rebound seizures or status epilepticus if omitted.

Advice in the perioperative period

Elective and emergency surgery


Patients should be advised to take their regular medications on the day of surgery.

Abrupt withdrawal of any anticonvulsant drug should be avoided.

Ensure that the patient is maintained on a specific manufacturer’s product (see Further information).

Post-operative advice

Regular dosing of the patient’s usual oral medication should be re-established as early as possible post-operatively.

If patients are unable to take their regular oral medication post-operatively, consider using the intravenous route – see below.


Intravenous phenytoin

Intravenous phenytoin is error-prone throughout the prescribing, preparation, administration and monitoring process; all relevant staff should be made aware of appropriate guidance on the safe use of injectable phenytoin to reduce the risk of error1. Due to its narrow therapeutic index, it is recommended to monitor plasma-phenytoin concentrations. Refer to local guidance and seek specialist advice where necessary.

Dose equivalence and conversion

Preparations containing phenytoin sodium are NOT bioequivalent to those containing phenytoin base. If switching between products the difference in phenytoin content may be clinically significant. Care is needed when making these changes and plasma-phenytoin concentration monitoring is recommended. Seek specialist advice where necessary.

Interactions with common anaesthetic agents

For general information regarding the use of anaesthetic agents in patients with epilepsy see Antiepileptics overview.

Phenytoin metabolism is saturable: toxic concentrations of phenytoin can develop in patients given drugs that inhibit phenytoin metabolism even to a small degree. Phenytoin itself is also a potent enzyme inducer and induces the metabolism of many drugs.

Neuromuscular Blocking Drugs (NMBDs)

The recovery time from atracurium, cisatracurium, pancuronium and rocuronium appears to be reduced in those taking phenytoin long-term (more than 1 week) – anticipate the need to use a larger dose of NMBD, and expect an accelerated recovery. However, acute use of phenytoin can increase the effects of these drugs.

Phenytoin increases the effects of suxamethonium. Monitor the outcome with concurrent use – expecting an alteration in recovery time.


Phenytoin is predicted to decrease the exposure to alfentanil/fentanyl. Monitor concurrent use for alfentanil/fentanyl efficacy and increase the dose of opioid as appropriate.


Benzodiazepines potentially affect the concentration of phenytoin (toxicity has been seen) – monitor concentration and adjust dose. Phenytoin after long-term use might reduce benzodiazepine concentrations as a result of induction of hepatic drug-metabolising enzymes. Monitor for reduced efficacy of benzodiazepines and phenytoin toxicity; consider monitoring phenytoin concentrations. Additive central nervous system adverse effects (e.g. sedation) might occur.


Phenytoin is predicted to decrease the exposure to esketamine – monitor and increase the dose of esketamine as appropriate.

Local anaesthetics

Phenytoin is predicted to decrease the exposure to IV lidocaine/ropivacaine. The clinical relevance of reduced lidocaine levels is most likely to be small. The incidence of central toxic adverse effects may be increased if lidocaine is used concomitantly with phenytoin. Sinoatrial arrest has been reported in one patient – caution is needed when giving two drugs that have cardiac depressant effects.

The manufacturers suggest that the effects of topical prilocaine and lidocaine may be additive with drugs that can cause methaemoglobinaemia, they name phenytoin. Caution is warranted.

Interactions with other common medicines used in the perioperative period


Phenytoin greatly reduces the concentrations and effects of dexamethasone and, to a lesser extent hydrocortisone. Monitor concurrent use, increasing the corticosteroid dose accordingly. Consider giving an alternative corticosteroid if problems arise.


Phenytoin is predicted to decrease the exposure to buprenorphine and oxycodone – monitor and increase the opioid dose as necessary.

Phenytoin increases the production of the toxic metabolite of pethidine. Limited evidence suggests that phenytoin induces metabolism of pethidine and could decrease its analgesic efficacy. Bear the possibility of this interaction in mind.


Ciprofloxacin affects the concentration of phenytoin, although the clinical relevance is not thought to be significant. Bear this interaction in mind in the case of unexpected response to treatment; consider monitoring phenytoin concentrations. However, quinolones very occasionally cause convulsions and should generally be avoided in patients with epilepsy.

Phenytoin decreases exposure to doxycyline. It has been suggested that the doxycycline dose should be doubled.

Both phenytoin and metronidazole can increase the risk of peripheral neuropathy. There have been a few anecdotal reports of patients who have developed toxic phenytoin concentrations when given metronidazole. Monitor for signs of phenytoin toxicity (such as blurred vision, nystagmus, ataxia, or drowsiness).

Phenytoin serum concentrations can be increased by co-trimoxazole (contains sulfamethoxazole). Monitor phenytoin concentrations and adjust the dose accordingly.

Whilst single surgical prophylactic doses should not pose a problem, continued post-operative treatment may require close monitoring. Consult current product literature.


Bear in mind a potential interaction between phenytoin and prochlorperazine – there has been a single report that prochlorperazine has been noted to impair phenytoin metabolism. Monitor for unexpected response to treatment. However, see Antiepileptics overview for advice regarding the use of phenothiazine antiemetics in patients with a history of seizures.

Phenytoin is predicted to decrease the exposure to ondansetron. Be aware that ondansetron might be less effective.


Phenytoin decreases the exposure to paracetamol via increased metabolism. Consider an interaction as a possible cause of any reduction in paracetamol efficacy.

Further information

MHRA/CHM Advice: Antiepileptic Drugs: updated advice on switching between different manufacturer’s products (November 2017)

Phenytoin is a category 1 antiepileptic and patients should be maintained on a specific manufacturer’s product. For more information see Antiepileptics overview.


Joint Formulary Committee. British National Formulary (online) London: BMJ Group and Pharmaceutical Press [Accessed 30th June 2019]

Perks A, Cheema S, Mohanraj R. Anaesthesia and epilepsy. BJA: British Journal of Anaesthesia. 2012; 108(4):562-571

Carter EL, Adapa RM. Adult epilepsy and anaesthesia. BJA Education. 2015; 15(3):111-117

Phenytoin. In: Brayfield A (Ed), Martindale: The Complete Drug Reference. London: The Royal Pharmaceutical Society of Great Britain. [Accessed 30th June 2019]

Summary of Product Characteristics – Phenytoin 100mg film-coated tablets. Aurobindo Pharma – Milpharm Ltd. Accessed via 06/07/2019 [date of revision of the text June 2019]

Baxter K, Preston CL (eds), Stockley’s Drug Interactions (online) London: Pharmaceutical Press. [Accessed on 30th June 2019]

Atracurium. In: Brayfield A (Ed), Martindale: The Complete Drug Reference. London: The Royal Pharmaceutical Society of Great Britain. [Accessed 30th June 2019]

Suxamethonium. In: Brayfield A (Ed), Martindale: The Complete Drug Reference. London: The Royal Pharmaceutical Society of Great Britain. [Accessed 30th June 2019]