Just like any product that has a physiological effect on the human body, you can over-indulge. Similar in the way you can have too much caffeine or you can have too much alcohol, you can have too much cannabis.

Consuming too much cannabis may lead to a few unpleasant hours. However, unlike other commonly prescribed medicines for treating pain and other conditions approved for this program, it is highly unlikely that an otherwise healthy individual would experience a lethal reaction from over-consuming cannabis. This is principally because the cannabinoid receptors, unlike opioid receptors, are not located in the brain stem areas that control respiration and cardiovascular function. In the nearly 5,000 years that cannabis has been used by tens of millions of people for both medicinal and recreational purposes, there has not been one credible documented case of someone fatally overdosing on cannabis.

There is a test known as the LD-50 used to determine the toxicity of a product. This rating indicates at what dosage fifty percent of test animals receiving a drug will die as a result of drug induced toxicity. The LD-50 rating for aspirin is 1:20. in layman’s terms this means that if the recommended dosage of aspirin is two pills, in order to induce death, a person would need to consume 40 pills (20x’s the recommended dosage). For Valium it’s 1:10 and for some cancer medications it can be as low as 1:1.5.

It was estimated that the LD-50 for cannabis would be between 1:20,000 and 1:40,000; meaning that a person would need to ingest 20,000 to 40,000 times the single serving size to induce a lethal reaction. For this study, a single serving of cannabis was measure at .9 grams of cannabis (the amount in one cannabis cigarette or joint) – a person would need to ingest 20,000 to 40,000 joints, or roughly 1,500 pounds of cannabis in 15 minutes to induce a lethal reaction. Simply put, it would take an unrealistic amount of cannabis consumption for an otherwise healthy person to experience a cannabis-induced lethal event.

http://www.cancer.gov/about-cancer/treatment/cam/hp/cannabis-pdq#section/_11

http://norml.org/library/medical-marijuana-reports/item/marijuana-as-medicine-a-plea-for-reconsideration