Interactively, Ketamine should not be used with respiratory depressants, primarily alcohol, barbituates, and Valium.
One would presume that GHB falls into the category of respiratory depressants (or rather more correctly CNS depressants).
Whatever [GHB] side effects may be noted, they are often much more severe when GHB is combined with other central nervous system depressants [referenced]
This quote indicates that downers exacerbate the side-effects of GHB, as does the following quote from the Contraindications section of the same GHB FAQ.
GHB should not be used with benzodiazepines ("minor tranquilizers" such as Valium and Xanax), phenothiazines ("major tranquilizers" like Thorazine and Stellazine), various painkillers (barbiturates and opiates), alcohol, anticonvulsants (Dilantin and phenobarbital) and even many over-the-counter allergy and sleep remedies--without direct medical supervision.
It's very much a case of avoiding things that normally make you sleepy or slow you down, when taking GHB. Falling into an unrouseable sleep in public can be a nuisance, but sedating yourself to the point where you stop breathing is another issue entirely.
BigTrancer
I'd like to add that for both substances it is way too easy to go over the required doses, filling a measuring cup while you're hands are numb
Um, well ive never had trouble.. Spose it all depends on how stupid/careless you are.. Not saying ive never taken things to far.. But if i have, I intended to when i dosed.
Two depressants are never super safe together, but i dont think K (for me), seems to depress my breathing very much...It dissacociates.. ANyway.. Each to there own.. As i said, I wouldn't recommend anyone playing with this area, kinda like playing with benzo's and grog...