Without a doubt, Dramamine. I've done acid many times without any real long term complaints, though I once took 24 standard motion sickness pills and to this day (many months later) I have a small red dot in the center of my vision. It's not imposing, and it's not really noticable unless I look for it or zone out, but it's always there. There are other visionary effects that also continue to this day, albeit midly. Of course, the trip is horribly fucked up, and I couldn't make out much in terms of when I started and stopped tripping.
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I must live twice as hard and half as long