Tony Spinola (26) was caught in his cell with a small amount of the drugs during a routine search with a sniffer dog, and he immediately told officers where the stash had been hidden.
Spinola is serving four and a half years for possessing cannabis with intent to supply and being involved in the supply of the drug.
He admitted possessing about £70 worth of heroin and cannabis at the prison on 29 September last year.
Crown Advocate Sally Sharpe said that Spinola started using cannabis at 17 or 18, and developed an £80-a-week habit.
He occasionally took ecstasy too, but had not used drugs in the prison until six weeks before his arrest.
Defending, Advocate Rebecca Juste said that her client had been in the ‘drug-free’ wing for nine months before the heroin and cannabis was found.
He had gone through a ‘rough patch’ in coping with the length of the sentence he was serving, but emphasised that he had no intention of selling the drugs.
Advocate Juste added that Spinola had been obliged to wait four and a half months to find out whether he would be charged over the heroin and cannabis, and he was only told on the morning before he was taken to court.
The court jailed Spinola for an additional five months.
The Bailiff, Sir Philip Bailhache, told him that being caught with the drugs in the ‘drug-free’ wing was an aggravating factor, but they had shown leniency because of the delay in bringing the case to court.
The Bailiff was sitting with Jurats Le Brocq and Allo.
