How Many Dosages Of THC Must A Human Take To Reach The LD-50% Amount?
Posted October 08, 1999 at 23:06:41 PT
Written by Pat Whelan

Cannabis Calculations - How Many Dosages Of THC Must A Human Take To Reach The LD-50% Amount?


A year or so ago while I was surfing the web I thought I read somewhere that the Therapeutic Index of THC was 40,000. I think the Therapeutic Index is sometimes called the Safety Ratio, and is the number you get if you divide the LD-50% dose of something, by it's normal dose. I think the LD-50% dose is a statistical term which means the dose or amount of a substance that will kill about 50% of the test subjects. 

Another way to look at the Therapeutic Index is this - it simply states how many normal-sized doses of a certain substance a given population of animals has to take, in order to reach the level that kills about 50% of them. So, in general, the higher a T.I. or Safety Ratio something has, the safer it is to use as this means there is less chance of dying from an overdose if one takes too much. 

The 40,000 number above for THC's T.I. seemed so very high to me compared to the T.I. quoted for any drug I've seen that after awhile I began to doubt my memory. For example, a legal drug might be considered a really good safe drug, with a comfortably high T.I., if it's T.I. is between say, 20 and 30, and if it's use confers some beneficial effect or effects. So, I started a little 'net quest to find out what the real truth is about THC. At the time I found that most sources quoted the Therapeutic Index for THC as a number between 20,000 and 40,000, but the lowest number I found was just a little over one or two thousand, (for one type and one sex of mouse), and the highest was 80,000. These numbers left me with some questions, such as: is the T.I. for small rodents the same as it is for humans? And, did the researchers by chance kill the small rodents by busting their stomachs when they filled them up with their vegetable oil-THC mixture, or did they suffocate them by filling their lungs up the oil? So… I kept searching the Internet until I found an Australian web site with the most important piece of info that I was seeking.

The LD-50% Dose for THC in Dogs and Monkeys.

The researchers who did the Australian research found out that when they gave dogs and monkeys THC intravenously, that the LD-50% dose was much higher for these animals than other researchers had previously found in their studies of small rodents. They stated in their report that it was assumed that humans are at least as resistant to the effects of THC as monkeys are, so they inferred that their numbers are similar to numbers that would be found if this study was done on humans.

Because this is so, they went on to state that the inferred LD-50% dose of pure THC for a 65 kg human would be the (astounding) number: 8.45 kg.

But… they did not then go on to compute what the approximate true (inferred) Therapeutic Index of THC would be for humans. So, I did….

I found a web site which stated the normal dose of smoked THC for humans was about 2-22 milligrams; I rounded these numbers off to 2-20 mgs. I also inferred that the effect of smoking THC was about equal to injecting it directly into your veins. Very recently, I found an Internet source which states that THC has it's strongest effect when it is injected directly into your veins, rather then when it is smoked, so I guess my assumption above, is a safe one to make.

Computing the inferred Therapeutic Index of THC for Humans.

8450 divided by 0.020 = 422,500 and…
8450 divided by 0.002 = 4,225,000

Since only the first one or two most significant numbers above has any precise or true meaning, we see that the true Therapeutic Index for THC in humans varies from about 400,000+ to 4,000,000+ or so. By the way, I think I saw a source recently which stated that for some folks as little as 300 micrograms of THC would be enough to make them high. If we use this number to compute the Therapeutic Index of THC in humans…

8450 divided by 0.0003 = 28,166,666

we find that some folks would need to take as many as 28,000,000 million normal-sized dosages all at once to reach their LD-50% dose….


Therefore, we see that it takes from about 400,000+ to about 4,000,000+ to perhaps as many as 28,000,000 normal-sized dosages of THC - taken all at once and injected intravenously - for a human to reach the LD-50% level.

We see that THC compares quite favorably with alcohol, which takes as little as 4-10 normal-sized dosages to reach the LD-50% dose in humans. 

Animals studies show that a given amount of a natural mixture of cannabinoids from the cannabis plant has less adverse effects on these animals then does the same amount of plain THC alone does. I gather from this that probably all the cannabinoids are essentially non-toxic and that when combined, as they are in their natural state in the cannabis plant, that they can exhibit even more beneficial effects, and less detrimental effects than the same amount of THC alone does. 

I know of no other commonly used item with beneficial pharmacological effects which has a Therapeutic Index or Safety Ratio anywhere near as high as that of THC and it's other related cannabinoids. 


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