Most ketamine is racemic, i.e. it is composed of equal parts S/(+) and R/(-) ketamine. The (+) isomer of ketamine is more potent than the (-) isomer. It also seems that (+) ketamine would be superior as a psychedelic tool, for a number of reasons.
Some facts/differences worth noting:
In a study which used male adults, the amount of ketamine needed for total anaesthesia was ~271 mg (+) ketamine vs ~409 mg racemic ketamine. 
(+) ketamine is generally considered to be approx. 2-3 times more potent than (-) ketamine.
(+) ketamine is cleared from the body in HALF the time it takes for racemic ketamine. The clearance of (-) ketamine takes only slightly longer than racemic ketamine.
(-) ketamine significantly inhibits the clearance of (+) ketamine. This means that if you have pure (+) ketamine, after you come out of a k-hole, the aftereffects of the k will wear off more quickly than they would if you had done racemic ketamine.
(+) ketamine inhibits the dopamine transporter 8 times more potently than (-) ketamine.
[quote]However, application of S(+)-ketamine was associated with a remarkably smoother emergence period, a profound postoperative analgesia, a more rapid recovery of cerebral functions, and a greater preference by the study persons. The incidence of psychotomimetic phenomena appeared to be negligibly less after S(+)-ketamine in comparison to racemic ketamine, but their quality was described as far less unpleasant. Clinical use of S(+)-ketamine administered at one-half of the usual dose is thus not only associated with a reduction of undesirable adverse effects without altering ketamine's anaesthetic and analgesic potency, but also offers distinctive improvements due to the reduced drug load. Moreover, increasing experimental evidence supports a remarkable neuroprotective effect of S(+)-ketamine, which may become a promising drug for new therapeutic approaches to neuroprotection.
[quote]It was found that (S)-ketamine binds with a 3-4 time higher affinity to the PCP binding site of the NMDA receptor than (R)-ketamine, and that at these concentrations (R)-ketamine interacts also weakly with the sigma receptor sites, where (S)-ketamine binds only negligibly....R)-ketamine did not produce psychotic symptoms, but a state of relaxation. The (S)-ketamine-induced metabolic hyperfrontality appears to parallel similar metabolic findings in acute psychotic schizophrenic patients and encourages further investigations of glutamatergic disturbances in schizophrenia.
This leads me to believe that the psychedelic effects of racemic ketamine are being produced by the (+) isomer. This would further lead me to conclude that pure (+) ketamine would be more psychedelic in its subjective effects -- as well as less sedating (as noted in the study). 
With sub-anaesthetic doses of ketamine in humans, 50% of the test subjects who took racemic ketamine experienced anterograde amnesia (could not remember portions of the k-experience) , while only 8% of those who took (+) ketamine experienced amnesia. This means that your ability to recall a ketamine trip would be greatly improved if you were taking pure (+) ketamine. 
[quote]Subjective mood was judged by the volunteers to be significantly better after S-(+)-ketamine, and volunteers found S-(+)-ketamine to be more acceptable than racemic ketamine. The frequency of dreams was the same after both drugs. No unpleasant dreams were reported after S-(+)-ketamine, but one of the volunteers who received racemic ketamine had uncomfortable dreams. It seems that (+) ketamine has a distinct positive vibe to it, unlike racemic ketamine which is more neutral (or even negative) in its psychological effects. 
 Ihmsen H, Geisslinger G, Schuttler J.
Stereoselective pharmacokinetics of ketamine: R(-)-ketamine inhibits the elimination of S(+)-ketamine. Clin Pharmacol Ther. 2001 Nov;70(5):431-8.
 Nishimura M, Sato K. Ketamine stereoselectively inhibits rat dopamine transporter. Neurosci Lett. 1999 Oct 22;274(2):131-4.
 Himmelseher S, Pfenninger E.The clinical use of S-(+)-ketamine--a determination of its place Anasthesiol Intensivmed Notfallmed Schmerzther. 1998 Dec;33(12):764-70.
 Vollenweider FX, Leenders KL, Oye I, Hell D, Angst J. Differential psychopathology and patterns of cerebral glucose utilisation produced by (S)- and (R)-ketamine in healthy volunteers using positron emission tomography (PET). Eur Neuropsychopharmacol. 1997 Feb;7(1):25-38.
 Pfenninger E, Baier C, Claus S, Hege G. Psychometric changes as well as analgesic action and cardiovascular adverse effects of ketamine racemate versus s-(+)-ketamine in subanesthetic doses Anaesthesist. 1994 Nov;43 Suppl 2:S68-75.
Doenicke A, Kugler J, Mayer M, Angster R, Hoffmann P.[Ketamine racemate or S-(+)-ketamine and midazolam. The effect on vigilance, efficacy and subjective findings]
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