Liam Christopher Stevenson, an Otago Polytechnic student, died in the crash on State Highway 88 at Ravensbourne.
Before driving a friend's Mazda car, he smoked a small amount of cannabis from a bong while at a party in Ravensbourne, the inquest heard.
Others at the afternoon party on May 19 last year also smoked cannabis, as well as the synthetic cannabis known as Kronic, 18-year-old Liam Kennard said.
Mr Kennard was a friend of the dead teen and attended the Ravensbourne party.
Kronic was smoked by most at the gathering before Liam arrived with a bag of cannabis, Mr Kennard said.
"Liam Stevenson was happy, he was sober, he was fine. He smoked a bong ... the bong contained cannabis. It wasn't very much, enough to fill a cone-piece. He was fine after the bong. He was happy," Mr Kennard said.
Toxicology tests on Liam's body showed he had 3mcg of cannabis THC per litre, and 7mg of alcohol per millilitre of blood.
Kronic was not detected.
Mr Kennard said there was alcohol at the party, although he did not see Liam drink any.
Otago-Southland coroner David Crerar said the level of cannabis in Liam's blood was consistent with him having smoked the equivalent of a single cannabis cigarette between 30 minutes and four hours before his death.
The level of alcohol was equivalent to Liam consuming about one glass of beer, and was not enough to impair his driving, Mr Crerar said.
At the time of Liam's death, testing for Kronic was not sophisticated and there could be no conclusion as to whether the teenager consumed the synthetic drug as well as cannabis, Mr Crerar said.
Death resulted from the severe head injuries suffered in the collision.
Liam also had a ruptured stomach and fractured right femur, Mr Crerar said.
No mechanical faults which could have caused the collision were found in either vehicle involved in the crash, nor were there any anatomical abnormalities or anomalies which could have affected Liam's ability to drive, he said.
Truck driver David Murray (50), of Dunedin, was in no way at fault, Mr Crerar said.
Mr Murray was not seriously injured in the crash, and said he remembered the Mazda approaching, then a loud bang.
"It happened so fast I didn't have time to brake," he said.
In the case of Liam's death, a description of the adverse effect cannabis use had on drivers was "prophetic", Mr Crerar said.
"It is a lesson to his friends who may still be using cannabis.
"You don't smoke and then drive. It's just not worth it," Mr Crerar said.
Dangers of driving after using cannabis included taking longer to respond to events, reduced ability to think clearly and pay attention, distorted perception, difficulty thinking and problem solving, and loss of co-ordination.
Mr Kennard said he was in the Mazda driven by Liam shortly before the crash.
Liam overtook a vehicle on SH88, narrowly avoiding a collision with an oncoming truck, Mr Kennard said.
"I swore at him and told him to get back into his lane."
Although Mr Kennard saw Liam smoke cannabis before driving, he was happy to be a passenger in the car.
"I didn't say anything to Liam about him driving me.
"I don't have a licence, I'm disqualified, so I wasn't going to drive," he said.
Both wore safety belts.
Liam dropped Mr Kennard off at the George St overbridge then turned back towards Ravensbourne, where he crashed minutes later.
Constable Amie Manning, of Dunedin, said she arrived at the scene about 4pm. Liam was already dead. His right arm was underneath his seat belt and his hand was in the pocket of his jeans, where his cellphone was.
"I drew the inference he had only one hand on the steering wheel at the time of the crash," she said.
Mr Crerar said it could only be assumed Liam was reaching for his cellphone.
Whether such an act was disruptive could also only be an assumption, he said.
The coroner found Liam died from severe head injuries, caused when he crashed into a truck while affected by cannabis.
His formal findings will be released in due course.
