The primary concern for safe LSD use is mental stability. This is not limited to whether or not you have or have been diagnosed with a mental illness. Strength of psyche/character/soul is often put on the line, particularly with higher doses. A lot of things that are kept hidden, repressed, and so forth can be brought to light, sometimes unwillingly, while on LSD. This can be extremely disturbing or unsettling. At higher doses, disassociation is common, where a person is no longer in touch with reality in any way. This can be dangerous when unsupervised, as a person might unwittingly and/or through an altered thought process do something that would cause them harm (falling, jumping, encountering sharp objects, etc).
In my opinion, there has yet to be any significant evidence to suggest any direct physical damage to your body or mind from LSD use, even high dosage use. The issue is the fact that a strong LSD experience can be just that, strong. It can cause stress on the body and mind, and for some people can trigger syndromes akin to post-traumatic stress disorder (PTSD). In these cases a person may associates certain things encountered in the experience with a traumatic state, and these external cues can later trigger a similar feeling or even visual, auditory, or mental distortions. This is not from physical damage or lingering presence of the drug, but from damage to the psychological state.
At any dose, one should be prepared and understand the possibilities of the experience, and be ready to take it as its comes. A sober but understanding sitter is quite helpful for ensuring the safety of the tripper. One should know one's own limits, and not jump into high dosages lightly or before being comfortable with low doses. Hopefully, one can remember that the experience will end, eventually this can be difficult when in the thick of it and when perception of time is distorted). Even with ultra-high doses, in most cases the experience will not last more than 18-24 hours. Standard duration is 8-12 hours with duration increasing a bit as dosage goes up. Nearly all instances of people reportedly "tripping for days" was from the mistaken identity of a drug called DOB or STP, which lasts up to 24 hours and has been misidentified as LSD.
Many people have safely and effectively taken extremely high doses of LSD. Some did so unintentionally, some fully knowingly. Others have had extremely bad experiences on low doses. One must know one's own body and mind, and judge accordingly. Be responsible, be prepared. The imporant thing for someone who has taken a high dose of LSD is to help keep them calm and protect them from hurting themselves or others. Hospitals only help if a person is in serious physical danger or are freaking out to the point where they are likely to hurt themselves or others. A comfortable place with patient, understanding people can be the best thing to help a person through rough times of a bad LSD experience. See the Psychedelic Crisis FAQ for information about how to help someone through a bad trip. It is hard to give accurate, broad, sweeping reccomendations or suggestions, tet we'd like to avoid giving useless broad discouragements against any use. Hope this helps.
