In some cases, offenders are held on remand in Italy for over a year before their trial. The FCO warned that the "zero-tolerance" approach of some countries often results in strict penalties which can come as a shock to British travellers.
Will Middleton, consular director for southern Europe, said the consequences of being detained for a drug offence can be "devastating".
“Being sent to prison overseas, away from family and friends, is very distressing and even more so if you don’t speak the language," he said.
"We see people of all ages - from youngsters through to pensioners - who have lost their friends, their job, had to give up their studies or had their children taken into care and got into major financial difficulties because they did not think they would get caught."
The FCO has now launched a campaign, in conjunction with the charity Prisoners Abroad, to highlight the consequences of the use, possession and smuggling of drugs in Italy and around the world.
Offences that may carry cautions in the UK are often penalised with long prison sentences overseas, the FCO said. Some drug crimes can lead to even more severe penalties – 33 countries or territories enforce death sentences for drug offences.
Prisoners Abroad is currently supporting 84 Brits between the ages of 18 and 30 who are being held in foreign countries for drugs offences – 62 are yet to face a trial, according to the latest figures from the charity.
An earlier report by the FCO noted that while there was a significant drop in general arrest and detention cases between June 2012 and June 2013 globally, countries including Italy, Canada, the Netherlands and the United Arab Emirates saw an increase in cases.