Tighter rules are to be introduced to stop "health tourists" taking advantage of cheaper prescriptions in Wales, First Minister Rhodri Morgan has said.
Prescription from Wales will look different to those from England
From Friday, the cost of a prescription in Wales has been cut by £1 to £4 while in England it went up 10p to £6.50
Prescription charges will be scrapped altogether in Wales by 2007.
Wales-only, bilingual prescriptions will be introduced to stop people coming to Wales to obtain cheaper medication.
Visiting a GP's surgery in Barry, south Wales on Friday, Rhodri Morgan promised to stop patients travelling from England especially to obtain prescribed drugs.
"(Health tourism) isn't a problem yet, but it could be when the difference gets even starker next year or the year after," he said.
"We are bringing in, in the autumn, tighter controls."
At the moment, there is nothing to stop NHS patients from England travelling to Wales to obtain their prescribed drugs more cheaply.
But Mr Morgan said that under-25s have had free prescriptions in Wales for four years, and there was little evidence that young people from England were using the policy to their advantage.
An assembly government spokesman said tighter controls would most likely be in the form of a "Welsh" prescription.
This would be bilingual for the first time, and show that it had been obtained from a doctor based in Wales by a patient registered in Wales.
Welsh pharmacists would then be able to tell if an English patient was offering a prescription written in England, and charge them accordingly, he said.
Patients with a Welsh prescription would be able to receive their prescription anywhere in the UK, but only at lower cost, or later for free, if they took it to a pharmacist that had a contract with a Welsh local health board.
Mr Morgan said the policy would help encourage those on welfare benefits to return to work.
"If you have a cheaper prescription, it's much less of a disincentive to move from welfare to work," he added.
Welsh Conservative health spokesman Jonathan Morgan, who is the prospective parliamentary candidate for Cardiff North, said: "If you live along the border then there is every possibility you would come to Wales to get your free prescription.
"This is something the government should have thought of before embarking on this policy."
Plaid Cymru health spokesman Hywel Williams, the MP for Caernarfon, said his party supported free prescriptions, but criticised the "piecemeal" way the policy was being introduced.
Liberal Democrat health spokeswoman Kirsty Williams accused Labour of "slow progress" on the issue.