People who combine a foreign holiday with an operation are frequently left to deal with unpleasant complications, a Which? magazine survey suggests.
It may be cheaper, but it could cause you problems, the survey suggests
Nearly one in five (18%) of respondents had health problems following treatment, including infections, and 8% needed urgent NHS care on their return.
An estimated 80,000 UK residents opted for cheaper surgery abroad in 2006.
Many cosmetic surgeons say they have ended up repairing the damage from botched surgery carried out overseas.
The comparatively low prices of surgery and dentistry abroad are fuelling the dramatic rise seen in the past decade.
Which? said that a tummy tuck which would cost £4,000 in the UK could be carried out for half that price in Poland.
However, specialists say that flying into a foreign city for an operation means that patients are not properly assessed prior to surgery - and not given the right support afterwards.
The majority of the 299 people polled by Which? said they were very satisfied with their treatment, but more than a quarter said they had not received the right follow-up care after their procedure.
Among the 18% who reported problems were one patient who developed a severe infection after a tummy tuck operation, and another who said that she had been left leaking fluid following liposuction.
Which? is also concerned about the quality of advice offered to would-be patients - one firm called anonymously said a full knee replacement was not a risky procedure, and that the patient would not need to be followed up by a clinician on returning to the UK.
However half of all knee replacement patients need physiotherapy and all are reviewed as part of standard practice.
Neil Fowler, the Editor of Which?, said: "Medical tourists should do their homework before jumping on the plane, and avoid rushing back too quickly if they want to avoid potential problems.
"Ask the right questions beforehand, speak to UK health professionals and don't assume you'll have a safety net if things don't go according to plan."
More than half the members of the British Association for Aesthetic Plastic Surgery who answered a survey last year said they had seen three or more dissatisfied patients following surgery abroad.
Its president Douglas McGeorge said: "My experience with patients is that counselling is inadequate - the individuals have no idea of the standards of care in the country they are visiting and no knowledge of the abilities or experience of the surgeon.
"Follow-up is difficult - complications do occur and are usually left to the British system to treat."
Michael Summers, the vice-chairman of the Patients Association, said that patients should think very carefully before going abroad to have major surgery such as hip and knee replacement.
"There is a lot more to this than just saving money - frankly, a 50% satisfaction rate is just not good enough, and if you come back to this country with problems after surgery, the NHS doctor won't know exactly what has been done to you, won't have seen the x-rays.
"It's a frightening prospect."
The Which? survey was sent out to 200,000 people.