Nicotine replacement therapy (NRT) may be less effective at weaning smokers off their habit than previously thought, researchers say.
A patch is one of six available forms of NRT
A University of Geneva team looked at studies of 4,800 adults and found 30% of those who had used NRT were smoking again a year or more after quitting.
The problem, the researchers say, is that earlier studies have failed to follow up smokers in the long-term.
This latest study appears online in the journal Tobacco Control.
The researchers say the evidence for existing treatment guidelines has mostly been based on the impact of a single course of NRT treatment after six to 12 months.
This, they argue, fails to take account of the substantial numbers of people who revert to smoking at a later date.
The Geneva team analysed trials assessing the effect of NRT on a total of almost 4,800 adults.
They found relapse rates were fairly consistent. Between the one-year milestone and subsequent check-ups, almost a third (30%) of those using NRT resumed smoking.
The effectiveness of NRT fell from 10.7% of smokers quitting, compared with those given dummy treatment, at one year to 7.2% at an average of four years later.
Most people relapsed within the first two years of a single course of treatment.
Those who made it beyond that point, tended to substantially cut their chances of starting smoking again.
The aim of NRT is to wean smokers off tobacco for good, or for sufficient time to confer a substantial health gain.
Bearing this in mind, the researchers argue, the effect of NRT is "modest".
However, lead researcher Dr Jean-François Etter said: "NRT is one of very few effective drugs for tobacco dependence.
"What our analysis showed is that the effect of NRT fades away over time.
"We concluded that, like for any other chronic condition, the treatment of tobacco dependence should probably last much more than the usual eight to 12-week course currently given to smokers.
"Several treatment episodes over several years would probably be more appropriate."
Dr Etter said the best approach was probably to combine NRT with behaviour therapy.
He said: "Many people relapse because withdrawal symptoms may be present even a long time after quitting, such as depression or weight gain, and because chronic nicotine use causes long lasting changes in the structure of the brain - more nicotinic receptors are present in the brain of smokers."
Dr Lynne Dawkins, of London's Goldsmith College, agreed that the effect of NRT was modest, but she said it was currently the best treatment option.
She said: "With a current lack of superior approaches, maybe we should ask ourselves what we can do maximise its efficiency?
"This might include ensuring that it is taken in sufficiently high doses, for a sufficient time period and perhaps in combinations of two or more products."
A Department of Health spokesman said the government was committed to helping people to quit smoking.
Providing NRT was only one part of a strategy which included beefing up stop smoking services, and a clampdown on tobacco advertising.
"We currently have the lowest smoking rates in England on record and these indicate that the government is on track to meet the target of 21% smoking prevalence in 2010."
There are currently six NRT products available in the UK. They include gum, a patch, a nasal spray, and a tablet.
Previous research has suggested smokers are twice as likely to quit smoking by using NRT rather than willpower alone, and four times as likely if they combine it with support from the NHS Stop Smoking Services.