Shaun Jones, 14, "excelled" as a rugby player, his head teacher said.
A "fit and healthy" 14-year-old boy died 12 hours after taking tablets for acne, an inquest has heard.
Shaun Jones saw his doctor about spots on his back and shoulders, Cardiff Coroner's Court was told.
He was diagnosed with mild acne and given a prescription. But the drugs were out of stock at the pharmacy.
Shaun, of Pontypridd, who "excelled at rugby", was given different tablets, but died the morning after taking them. The inquest continues.
The hearing heard that 6ft (1.83m) Shaun had previously used Clearasil to treat his spots but went to the doctor after hearing his friends had been given medication for acne.
After being given a prescription, he went to his local pharmacy with his mother Clare, where they were told that drug was out of stock.
Shaun and his mother were told the pharmacist had spoken to the doctor, and he could be given different tablets which were exactly the same as the other medication.
Mrs Jones noticed there was no safety leaflet included in the box, but thought nothing of that at the time, the inquest heard.
Shaun took the medication with a glass of water just before going to bed at 2230 GMT on October 20 last year at his home in the Rhydyfelin area.
Little more than an hour later, he complained to his parents of shortness of breath and tightness in his chest.
Mrs Jones initially contacted an out of hours GP service but was told no doctor was available, the hearing was told.
When Shaun's condition deteriorated, he was taken to the Royal Glamorgan Hospital, Llantrisant, by ambulance early the next morning.
Despite treatment there, and at the University Hospital of Wales, Cardiff, where he was transferred, Shaun died at 1040 GMT.
Doctors at the University Hospital of Wales believe an "idiosyncratic reaction" to the acne medication could have caused his death.
In a statement read to the court, Mrs Jones said: "My son was fit and healthy. He was 6ft tall and physically very fit.
"He excelled at rugby and trained and played very hard. I'm mystified by his death as is my entire family."
She said she had since conducted her own research into the medication Shaun took.
Mrs Jones said: "Had there been instructions in the box and having read all the information on this drug since, I would have called 999 straightaway and certainly wouldn't have given him the tablet before going to bed."
The whole episode was "extremely painful" for her, Shaun's father Graeme and his younger sister, she said.
Dr Stephen Jolles, consultant immunologist at the University Hospital of Wales, told the inquest that it was possible either the colourings or the active ingredient in the Sebomin tablets Shaun took caused the reaction.
He said it was possible for only "tiny amounts" of a substance to cause a reaction if someone was allergic.
The drug Shaun was originally prescribed, which was out of stock at the pharmacy, was Minocin, the inquest heard.
Dr Rim Al-Samsam, consultant paediatrician at the hospital, said she did not believe an infection or anaphylactic shock had caused Shaun's death.
Shaun was described by James Williams, his head teacher at Hawthorn High School, Pontypridd, as an "extremely popular student" who "excelled" in playing rugby for his school, village and district teams.
In a statement read to the inquest he said: "He'd everything to live for and would have been a credit to himself, his family and the school.
"There is no reason for him not to have been not only an excellent rugby player but a valuable member of the community."
The inquest also heard from Christopher Marris of Actavis, the company which makes the drug Shaun was prescribed.
He said the warning leaflet that should have been in the packet did mention difficulty in breathing or swallowing and said, in those circumstances, a patient should consult a doctor or indeed go to hospital.
He also said there had been a small number of deaths from liver failure - two of them in 1996 - but not any cases of this kind.
The inquest was adjourned until Friday.