Rocuronium is a muscle relaxant that anaesthetists give in hospital during surgery. The leaflet lists phenytoin, an anti-seizure medicine, among the medicines that can interfere with how rocuronium works. This applies when phenytoin is given suddenly (an acute dose). The leaflet does not say here whether the effect is stronger or weaker. If you take phenytoin, tell your surgeon and anaesthetist before any operation. They can plan the doses of the anaesthetic medicines and watch you more closely.
These medicines are often taken together with some care. Your doctor may change a dose, space them apart or check on you more often. Make sure your doctor and pharmacist know you take both.