many drugs have no LD50- no fatally toxic dose, because the human body can only metabolize a certain amount at a time, and simply flushes the rest out. most psychoactive chemicals that people would want to use recreationally are simply foreign neurotransmitters that react only physically, and not chemically with the human body, by fitting into a receptor. this is different than ingesting some toxins, such as, say a caustic chemical which would rip apart the body on a molecular level. the main two things to worry about in terms of overdoses of neuropharmiceuticals are 1)neural heat shock and overworking of neruons (built up waste material can eventually tear apart the cell) and 2) impaired body functions- some drugs effect the deeper areas of the brain that control the simple routine controls of the body, ie blood pressure, heart rate, breating, etc. and if you take to much of them one of those can get really high, ie high blood pressure causing a stroke, or they can get so low that they turn off, i.e. heart stops beating. there is a third thing to worry about with drug use that is not entirely an 'overdose' problem, but is another way in which they can severely damadge your brain or even kill you, and that is the question of siezure activity. many drugs can cause siezures, and some cause them in their withdrawal- ironically these are usually siezure medications.