Phenytoin is extensively bound to serum plasma proteins and is prone to competitive displacement. Phenytoin is metabolized by hepatic cytochrome P450 enzymes CYP2C9 and CYP2C19 and is particularly susceptible to inhibitory drug interactions because it is subject to saturable metabolism. Inhibition of metabolism may produce significant increases in circulating phenytoin concentrations and enhance the risk of drug toxicity. Phenytoin is a potent inducer of hepatic drug-metabolizing enzymes. Serum level determinations for phenytoin are especially helpful when possible drug interactions are suspected.
After oral administration, ciprofloxacin is widely distributed throughout the body. Tissue concentrations often exceed serum concentrations in both men and women, particularly in genital tissue including the prostate. Ciprofloxacin is present in active form in the saliva, nasal and bronchial secretions, mucosa of the sinuses, sputum, skin blister fluid, lymph, peritoneal fluid, bile, and prostatic secretions. Ciprofloxacin has also been detected in lung, skin, fat, muscle, cartilage, and bone. The drug diffuses into the cerebrospinal fluid (CSF); however, CSF concentrations are generally less than 10% of peak serum concentrations. Low levels of the drug have been detected in the aqueous and vitreous humors of the eye.
Since September 2012, the marketing licence in the UK has been held by Flynn Pharma Ltd, of Dublin, Ireland , and the product, although identical, has been called Phenytoin Sodium xx mg Flynn Hard Capsules. (The xx mg in the name refers to the strength—for example 'Phenytoin sodium 25 mg Flynn Hard Capsules').  The capsules are still made by Pfizer 's Goedecke subsidiary's plant in Freiburg , Germany and they still have Epanutin printed on them.  After Pfizer's sale of the UK marketing licence to Flynn Pharma, the price of a 28-pack of 25 mg phenytoin sodium capsules marked Epanutin rose from 66p (about $) to £ (about $). Capsules of other strengths also went up in price by the same factor—2384%,  costing the UK's National Health Service an extra £43 million (about $ million) a year.  The companies were referred to the Competition and Markets Authority who found that they had exploited their dominant position in the market to charge “excessive and unfair” prices.