According to the FDA there have been at least 279 deaths where medical marijuana was at least a secondary cause.
There were at least 180 deaths where non medical marijuana was at least a secondary cause of death.
There have been 92 deaths where synthetic marijuana was at least a secondary cause of death. These only include the ones reported to the FDA.
So yes you can say there has not been an overdose of marijuana and there has been an overdose of say water but to imply that marijuana has no adverse health impact or has no relation to mortality would be inaccurate.
Top recipients of pharmaceutical lobbying cash. 2009-2010
Burr, Richard (R-NC)Senate $227,119
Reid, Harry (D-NV)Senate $210,350
Specter, Arlen (D-PA)Senate $182,200
Hatch, Orrin G (R-UT)Senate $170,565
Eshoo, Anna (D-CA)House $168,350