There has never been a single recorded case of a toxic reaction with any component of cannabis. Neither in humans or laboratory primates. There is no proven toxic dose (LD-50) because scientists could not kill a single monkey with it. Even when infusing the body with THC intravenously.
Some rabbits and mice (not monkeys) were killed but the equivalent dose in humans would be 3-5 lbs of dry cannabis in one sitting (more than a daily smoker would consume in a lifetime). There is also evidence that those animals were not killed by cannabis but instead by too much dilution in their blood.
Cannabis is probably the safest thing you can overconsume. You have more toxic plants in your spice cabinet. Eating too much tuna is toxic. Even water is toxic when overcsonsumed.
Is THC the only ingredient?
There have been lots of stories about people hospitalized after being given brownies and stuff laced with pot.