As the others already stated, Zyprexa will severely diminish the activity of LSD since their mechanisms of action are direct opposites:
* Classic Psychedelics in general primarily act as agonists at the 5HT2A and 5HT2C serotonin receptors, with LSD in particular also agonizing a wide range of other serotoninergic, dopaminergic, and adrenergic receptors.
* Modern "atypical" Antipsychotics in general primarily act as antagonists at 5HT2A and 5HT2C serotonin receptors, with Zyprexa in particular also antagonizing a very wide range of other serotonergic, dopaminergic, adrenergic and other receptors.
Serotonergic antidepressants (SSRI's, SNRI's, tricyclics), too, tend to diminish psychedelic effects, although not nearly as strongly as antipsychotics.
The problem with hallucinogens is that they are capable of so much more than just causing spectacular visual and auditory hallucinations - in the early days, they sometimes used to be called "psychotomimetics" as they can indeed cause states of significant emotional dysregulation where your own feelings of isolation, frustration, doubt and despair are brought to the forefront of your mind and greatly amplified; this can be very valuable under the guidance of a therapist, but also extremely dangerous when unprepared.
Seriously, read, for example, the experience reports on DOM/STP in "Pihkal":
There was the magnification of light, color and odors. It was all very pleasant and beautiful, except that I had an overwhelmingly negative feeling. This at times grew to considerable intensity, and I feel it was clearly due to anger. At times the negativity disappeared completely, and I broke into the most enjoyable, even hilarious experiences. I alternated about 50-50 between joy and discomfort. As the evening drew on, I became withdrawn and pensive. It seemed clear that I had made all the wrong decisions--choice of partner, place to live, isolation, no meaningful activity. The greatest shocker was that my practice of meditation, which is one of my central focuses, and which I thought had brought me much peace and understanding, seemed to be a delusional solution to my unhappiness and isolation. The experience continued unabated throughout the night with much tension and discomfort. I was unable to get any sleep. I hallucinated quite freely during the night, but could stop them at will. While I never felt threatened, I felt I knew what it was like to look across the brink to insanity.
Not trying to be snobbish here - I myself absolutely underestimated these aspects of the psychedelic experience too, as well as the steep dose-response curve of some of these substances (ex.: 225 µg of 1P-ETH-LAD was exponentially more intense than 150 µg).