Like it's distant cousin DXM, Ketamine is metabolized by the cytochrome P450 enzyme. This means a nice big glass of grapefruit juice 1/2 - 1 hour before ketamine administration will potentiate the effect and increase the duration of activity. Although this is not really desirable with DXM in my opinion, ketamine is so short acting and expensive that getting the extra bang is well worth it.
One study demonstrating the link can be found here:
http://www.ncbi.nlm.nih.gov/entrez/q...cmd=Retrieve&a mp;db=pubmed&dopt=Abstract&list_uids=11353758&it ool=iconfft&query_hl=3
Cheers,
Tark