Phenytoin Compatibility

• Phenytoin has a pH in the range of 10 - 12.3. It will only stay in solution when the pH is considerably alkaline (about 10 - 12). 
• The mixing of phenytoin sodium injection with other drugs is not recommended

Compatible diluents	Incompatible diluent
0.9% sodium chloride Lactated Ringer's injections	Dextrose containing solutions as it provokes significant precipitation of phenytoin sodium injection via interruption of the cosolvent effect of propylene glycol. crystals were observed in all dextrose solutions after dilution. factors other than pH and cosolvent concentration may affect phenytoin stability in dextrose solutions.

Administration

• increased risk of adverse cardiovascular reactions associated with rapid administration, IV should not exceed 50 mg per minute in adults. 
• In pediatric patients, the drug should be administered at a rate not exceeding 1-3 mg/kg/min or 50 mg per minute, whichever is slower
• As non-emergency therapy, Phenytoin Sodium Injection should be administered more slowly as either a loading dose or by intermittent infusion. 
• Because of the risks of cardiac and local toxicity associated with IV phenytoin, oral phenytoin should be used whenever possible
• Because of the risk of local toxicity, IV Phenytoin Sodium Injection should be administered directly into a large peripheral or central vein through a large-gauge catheter. 
• Patency of the IV catheter should be tested with a flush of sterile saline. 
• Each injection of parenteral Phenytoin Sodium Injection should then be followed by a flush of sterile saline through the same catheter to avoid local venous irritation due to the alkalinity of the solution

Storage:

• Store at 20°-25°C, excursions permitted to 15°-30°C
• The diluted infusion mixture (Phenytoin Sodium Injection plus normal saline) should not be refrigerated. 
• If the undiluted parenteral Phenytoin Sodium Injection is refrigerated or frozen, a precipitate might form: this will dissolve again after the solution is allowed to stand at room temperature. 
• The product is still suitable for use. A faint yellow coloration may develop, however this has no effect on the potency of the solution.

References:

1. http://www.drugs.com/pro/phenytoin-sodium.html
2. https://www.medicines.org.uk/emc/medicine/27054
3. http://www.ncbi.nlm.nih.gov/pubmed/22223379