Anonymous #2 said:

Just curious, are you trying to avoid killing someone or ensure it?

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and try a gram first and see what happens?
Never done heroin so I don't know

Coke, depending on how pure but a lot I assume.

According to a paper published in the journal Pharmacology, Biochemistry, & Behavior, the LD50 of Cocaine when administered IP in Mice was 95.1 mg/kg. The reference for this data is:

Bedford JA, Turner CE, Elsohly HN. Comparative lethality of coca and cocaine. Pharmacol Biochem Behav 1982 Nov; 17(5):1087-8.

More information about the measurement of LD50s for recreationally used psychoactives can be found at:

http://www.erowid.org/psychoactives/health/psychoactives_ld50s.shtml

peace,
sage

Heroin:
Some sources quote the median lethal dose (for an average 75 kg opiate-naive individual) as being between 75 and 375 mg

http://www.venture17.com/blog/2011/09/16/WHY-NOT-TRY-FENTANYL-HEROIN-Part-II-.aspx