Drugs are not uncommon in Fiji, including marijuana and amphetamines. All are illegal. The consequences if you are caught in possession can be serious – it is not uncommon for drug users in Fiji to be imprisoned in the psychiatric hospital.
Most travellers manage to avoid any run-ins with the local authorities. If you are arrested, though, you have the right to contact your embassy or consulate, which will be allowed to provide you with legal representation but can do little else.