Holidaymakers are being warned to check the rules on carrying medicines abroad to avoid falling foul of local laws.
The Foreign and Commonwealth Office (FCO) said some commonly-prescribed medicines were "controlled drugs" in certain countries.
In Japan, some cold remedies are banned while some sleeping pills require a licence in Singapore.
Travellers could risk a fine or even imprisonment if they break the rules, the FCO said.
The Foreign Office said it was becoming more popular to travel to countries further afield.
But according to a survey of 2,000 adults in the UK, only 33% of them would seek advice on medication rules before they travel.
Nearly half the population of the UK is on prescribed medication, meaning that around 21 million people could be risking difficulties.
Banned in Japan
Medication containing pseudoephedrine - found in over-the-counter medicines like Sudafed and Vicks - is banned in Japan.
And in Qatar, over-the-counter medicines such as cold and cough remedies are controlled substances and must be accompanied by a prescription.
Diazepam, Tramadol, codeine and a number of other commonly-prescribed medicines count as "controlled drugs" so the advice is to check the regulations in the country you wish to visit.
Failing to comply may result in arrest, a fine or imprisonment in many countries, including Greece and the UAE.
Other notable restrictions include:
- sleeping pills, anti-anxiety pills and strong painkillers require a licence in Singapore
- Costa Rica and China require visitors to bring a doctor's note with their prescribed medication
- in Costa Rica, you should only take enough medication for the length of your stay, with a doctor's note to confirm that this is the right amount
- in Indonesia, many prescription medicines such as codeine, sleeping pills and treatments for ADHD are illegal
- tourists should always carry a doctor's note with any personal medicine when visiting China
The FCO said anyone travelling this summer should visit their GP at least four to six weeks before their holiday to check if any of their prescribed medication contained "controlled drugs" such as codeine.
Countries such as India, Pakistan and Turkey have a list of medicines they will not allow into the country.
The FCO recommends contacting the embassy, high commission or consulate in the UK of the country you're travelling to for advice on the legal status of specific medications.
The gov.uk website has a full list of foreign embassies in the UK.
Tips for travelling with medicine:
- carry medicines (including those bought over the counter) in their correctly labelled container, as issued by the pharmacist, in hand luggage
- consider packing a spare supply of medication in the hold luggage in case of loss of hand luggage
- a letter from the prescriber detailing the medicines with the generic names for the medications can be helpful for border control checks, and in case medicines have to be replaced or medical help is required
- carry a note from the prescribing physician on letterhead stationery for controlled substances and injection medications
- take out an appropriate level of travel health insurance including repatriation and specific cover for any pre-existing illnesses