||The physical danger of ingesting LSD at normal doses is quite low. There are two reported deaths from unusually high doses documented in medical journals.|
The first, which is mentioned in Psychedelics Encyclopedia by Peter Stafford, is discussed in a 1977 Journal of the Kentucky Medical Association article. In this case, "the quantity of LSD in the blood indicated that 320 mg (320,000 micrograms) had been injected intravenously" under the mistaken idea it was speed. The amount (320mg) was an estimate by the authors of the article and not based on direct knowledge of how much was taken.
The second reported death, for which there is toxicological data but no details about the circumstances leading up to the death (ingested dose is not known, though post-death blood levels are), is documented in a 1985 Forensic Science International paper. Unfortunately, so little is documented in the article that it is hard to say much about this event.
There are a handful of other reports of serious problems (but not death) caused by very large doses. One person who took 40 mg survived. Another report outlines the case of a group of eight individuals who snorted large quantities of powdered LSD on the mistaken assumption that it was cocaine... all survived. See the LSD Dose page for details.
Estimates of the half-lethal human dose -- the dose at which half of the participants would die -- vary widely, with different calculations extrapolated from animal data putting it anywhere from 12 mg to 5+ grams.
Massive overdoses are possible only in cases where a person has access to manufacture- or distribution-level amounts of material; the average LSD user is unlikely to ever come in contact with such high doses.
Fysh RR, Oon MC, Robinson KN, Smith RN, White PC, Whitehouse MJ. “A fatal poisoning with LSD”. Forensic Sci Int. 1985;28(2):109-13.
Griggs EA, Ward M. “LSD toxicity: a suspected cause of death”. J Ky Med Assoc. 1977;75(4):172-3.
11/9/2000: First answered by Fire.
8/8/2007: Answer updated.