The Capital and Coast District Health Board says there was a possibility of harm to Alex Renton after cannabidiol was administered to him by his mother without doctors' knowledge.
Rose Renton told TV3 programme 3rd Degree she gave her 19-year-old son some Elixinol oil sent to her by a mother elsewhere in the country while he was in intensive care, and before it had been given the official sign-off.
The teenager was given permission to receive the product while in an induced coma in Wellington Hospital.
He was suffering from status epilepticus - an acute, prolonged epileptic seizure - and, despite the treatment, died earlier this month.
The DHB said staff were aware that Rose Renton had applied cannabidiol products to Alex's skin and she agreed to stop when asked about this.
However, staff were not aware that she subsequently administered her own cannabidiol oil to Alex orally via a syringe in his mouth or nasogastric tube.
The DHB said the oil Ms Renton used could have interacted with some of the anti-epileptic drugs used to prevent Alex's seizures, because they were metabolised by the body in a similar way.
It said doses of those anti-epileptic drugs were reduced before the approved Elixinol treatment began to avoid a potential interaction.
"Over the timeframe Rose admits to having administered her own treatment, staff had not made any dose reductions because they were unaware of her actions, so there is a possibility of harm."
Ms Renton said she did not calculate the dose sizes and the DHB have no knowledge of how many times it was administered.
The type of oil used and its cannabidiol component were also unknown, so it would not be possible to determine what effect it had on his treatment.
The founder of Elixinol, the company which produced the oil, said he was shocked to learn the teen's mother had been administering it to him before it was officially approved.
Paul Benhaim said his company was in contact with Ms Renton before the official approval.
"I understand the mother's plight, and the reasons why she may have done that, on the one hand and on the other hand, sure, I feel disappointed that we weren't privy to such information as it happened."
He said he was not at liberty to say whether Ms Renton's actions could have had any adverse effects on her son.
Associate Health Minister Peter Dunne called Ms Renton's actions "rather foolish" but understandable in the circumstances.
He granted her one-off approval to use cannabidiol on compassionate grounds, after a campaign by Ms Renton and others.
The minister said his approach would not have changed, even if he had known Ms Renton was administering the product.
"In many senses, I think my decision to approve Elixinol for her son was made in spite of her endeavours."
With regard to other possible requests that may come to use Elixinol, Mr Dunne said each case would be decided on its merits.
"I'll certainly be seeking advice from the Ministry of Health about the implications in terms of good patient management for the future.
"But as far as the medicinal cannabis issue goes, my decisions are going to [be] based on the facts, the evidence, and certainly not be based on emotion."
He said Ms Renton's advocacy was on behalf of her son and "has absolutely no wider impact on me beyond that."
Mr Dunne said he was not in a position to comment on how she was able to give her son the treatment.