Both Dilantin ( Phenytoin) 100mg and Dexamethasone are S4 meds. Both are non-addictive, not controlled meds.
Dilantin is used as an anticonvulsant, but as with many anticonvulsants which has "mood stabiling effect", Dilantin unfortunately is used less for this purpose. ( In Wiki, it is stated bcoz of the funding for research to Parke Davis co. was stopped back in past years).
Dexamethasone is a corticosteroid, used for inflammation. ( It has a similar indication as NSAIDS Ibuprofen, Napoxen, Diclofenac, etc). If suffering from joint pain, associated with inflammation, usually GP gives this to his patient, however NSAIDS are being used more as they are sold OTC , like Nurofen, Voltaren, Naprogesic, etc.
I take Dilantin early in the morning, both with codeine phosphate. It works for me. Providing Analgesia. As for Gluthetimide, it is a sleeping agent, used in the States, I think it is a controlled substance there.
Dilantin has an antianxiety effect I notice, as it stabilised the "hyper excited electrical conduct" in the brain. And it is non sedating, unlike other anticonvulsant. In fact, it is in the book Smart Drugs 1 by Dean Ward and Morgenthaler, Dilantin is used as a smart agent, usually by students, just before facing an exams. As during the exams, we feel a bit nervous, tense, think too much, leading to "blank focus", Dilantin, I noticed, helps a lot with this to think clearly and focus.
And it also stabilised heart rhythm as Wiki stated. ( when I drink too much coffee, I notice, arrhythmias, just by taking 1 Dilantin, in 1 hour, my heart rhythm comes back normal. it is actually a very helpful meds , as for me.)
So I suggest take Dilantin at the same time with Codeine, I notice it better than separating the two doses.