Holidaymakers are being warned they can be thrown in jail in many countries around the world if they are found with even a small quantity of drugs.
According to the Foreign Office, there are more than 850 Britons locked up in prisons for drug-related offences.
Many are detained for months without trial and face distressing living conditions, it said.
Launching a campaign with the charity Prisoners Abroad to highlight the consequences of the use, possession and smuggling of drugs it said: "The zero-tolerance approach of some countries often results in strict penalties which can come as a shock to British travellers.
"Offences that may carry cautions in the UK are often penalised with long prison sentences when overseas. Some drug crimes can lead to even more severe penalties, and in 33 countries or territories some drug offences carry the death sentence."
It warned that some countries keep suspects on remand for years before their cases are heard. Prisoners Abroad is currently supporting 80 Britons aged 18 to 30 held in foreign countries for alleged drugs offences. Two-thirds of these are still waiting trial and the others are serving sentences from one to nearly 39 years.
