No one has ever died of marijuana overdose by itself. But it is possible to suffer serious effects of too much THC. Cannabis growers continually find ways to increase the THC content of marijuana, so that the average THC content has increased from an average of 4% in 1983 to an average of 10% in 2009. Some strains of cannabis tested much higher. High doses of THC have been shown to result in mental confusion, panic attacks, and hallucination.