LD50 means 'Lethal dose -50% kill rate" to put it simply. It means its the dose at which minimum 50% of test subjects are killed. Thats it in a nutshell.

Here is a list of LD50's I ripped off WIKI

Substance    Animal, Route    LD50    Reference
Water    rat, oral    90 mL/kg (~90,000 mg/kg)    [7]
Sucrose (table sugar)    rat, oral    29,700 mg/kg    [8]
Vitamin C (ascorbic acid)    rat, oral    11,900 mg/kg    [9]
Grain alcohol (ethanol)    rat, oral    7,060 mg/kg    [10]
Table Salt    rat, oral    3,000 mg/kg    [11]
Paracetamol (acetaminophen)    rat, oral    1,944 mg/kg    [12]
THC (main psychoactive substance in Cannabis)    rat, oral    1,270 mg/kg males; 730 mg/kg females    [13]
Aspirin (acetylsalicylic acid)    rat, oral    200 mg/kg    [14]
Caffeine    rat, oral    192 mg/kg    [15]
Nicotine    rat, oral    50 mg/kg    [16]
Strychnine    rat, oral    16 mg/kg    [17]
Sodium cyanide    rat, oral    6.4 mg/kg    [18]
Aflatoxin B1 (from Aspergillus flavus)    rat, oral    0.048 mg/kg    [19]
Dioxin (TCDD)    rat, oral    0.020 mg/kg    [20]
Batrachotoxin (from poison dart frog)    human, sub-cutaneous injection    0.002-0.007 mg/kg (estimated)    [21]
Polonium 210    human, inhalation    0.00001 mg/kg (estimated)    [22]
Botulinum toxin (Botox)    human, oral, injection    0.000001 mg/kg (estimated)    [23]

Hydrocodone: 375 mg/kg

Alcohol: 10.6 g/kg

THC: 1270 mg/kg (No one has ever OD'd on weed)

Nicotine: 50 mg/kg

Vitamin C: 11.9 g/kg

Caffeine: 192 mg/Kg

Cocaine: 95.1 mg/kg

DMT: 110 mg/kg

5-MeO-DMT: 115 mg/kg

MDMA: 350 mg/kg

Codeine: 427 mg/kg

Mescaline: 880 mg/kg

Diphenhydramine: 500mg/kg

Morphine: 461 mg/kg

Alprazolam: 331 mg/kg

Phenobarbital: 176 mg/kg

Diazepam: 720 mg/kg

Methqualone: 326 mg/kg

Psilocybin: 280 mg/kg

Methamphetamine: 55 mg/kg

Methylphenidate: 350 mg/kg

Zolpidem: 695 mg/kg

Aspirin: 1500 mg/kg

Acetaminophen: 500 mg/kg

Fentanyl: 0.03mg/kg

Ketamine: 500 mg/Kg

GHB: 1100mg/kg

Dextroamphetamine: 96.8 mg/kg

Methadone: 30 mg/kg

PCP: 130 mg/kg

Codeine: 427 mg/kg

Mescaline: 880 mg/kg

Diphenhydramine: 500mg/kg

Morphine: 461 mg/kg

Alprazolam: 331 mg/kg

Phenobarbital: 176 mg/kg

Diazepam: 720 mg/kg

Methqualone: 326 mg/kg

Psilocybin: 280 mg/kg

Methamphetamine: 55 mg/kg

Methylphenidate: 350 mg/kg

Zolpidem: 695 mg/kg

Aspirin: 1500 mg/kg

Acetaminophen: 500 mg/kg

Fentanyl: 0.03mg/kg

Ketamine: 500 mg/Kg

GHB: 1100mg/kg

Dextroamphetamine: 96.8 mg/kg

Methadone: 30 mg/kg

Diphenhydramine: 500mg/kg

LSD : 12,000 ug => 12mg/kg

Clonazepam: > 4000 mg/kg

Iron (often used as a nutritional supplement): 1250 mg/kg

Ambien: limited info, anecdotal report of recovery from ingestion of 400 mg



Links...
http://www.asanltr.com/newsletter/02-2/articles/Neurotoxins.htm
http://www.erowid.org/psychoactives/health/psychoactives_ld50s.shtml
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/L/LD50.html
http://paipm.cas.psu.edu/1024.htm
http://delloyd.50megs.com/hazard/LD50.html
http://web.singnet.com.sg/~chuaeecc/venom/rpotent.htm
http://www.bnl.gov/esh/shsd/Programs/Program_Area_Chemicals_Highly_Acute_Toxins.asp
http://www.ilo.org/public/english/protection/safework/cis/products/safetytm/toxic.htm
http://mv.lycaeum.org/mu/LD50_list.html

More once I have time. Sorry for duplicates in the list - it was compiled from many sources.

More work is on its way ASAP, once I have time. I may invest in a few rats to do *cough* animal tests on. Also - they are rats! Rats need extermination! I may do a few 'red mist' tests on the corpses to relieve my anger and depression too!

another example is safrole, I think? It causes cancer in rats, but not in humans. Rats use a different metabolic pathway that I think leads to some form of an epoxide, while we do not. Is this true?

He didn't actually kill the rats -- he just got the data, and I think the data can be of some use..

Oh you are right! I missed that part. Good call.