Some veterans have reported symptoms such as fatigue and stress
Multiple vaccinations probably did not cause illness among British troops deployed in Iraq, research suggests.
The King's College London study concluded that troops tended to blame poor health on multiple jabs, even when they had not had them.
Several studies have linked ill health among troops to vaccinations.
But writing in the British Medical Journal, psychologist Dominic Murphy said those studies were flawed as they relied on troops' own recollections.
He said there was no association between multiple vaccinations and poor health when the jabs were recorded objectively from medical records.
Many British and US troops sent to Iraq have complained of poor health, such as fatigue, stress and depression - similar to veterans of the 1991 Gulf War.
Suspected possible causes include exposure to pesticides, uranium, nerve gas, other chemicals - and vaccinations.
The King's team randomly selected nearly 5,000 men and women deployed to Iraq since 2003.
The veterans who were most likely to report ill health were those who recalled having two or more vaccinations in a single day.
But when the researchers checked the health records of 10% of the veterans selected at random they found that although 70% reported receiving one or more vaccinations on a single day, the records showed only 43% actually got that many at once.
Dr Murphy said the findings suggested that people who were unwell were more likely to incorrectly recall having multiple jabs than people who were well - possibly because they are casting around for an explanation for their ill health.
He said: "We are not disputing they are ill. Deployments are obviously very stressful times, and this may well have resulted in the ill health of certain veterans."
"Following most major wars, going back to the Boer War, we have found evidence of ill health in military personnel who have served in those wars."
A large US review in 2006 showed that troops deployed during the 1990-1991 Gulf War get sicker than most other veterans, but the research did not turn up evidence of a common "Gulf War Syndrome".
British troops sent to Iraq are routinely vaccinated against anthrax, tetanus, typhoid and yellow fever.
Major Charles Heyman, editor of Armed Forces of the UK, said there was no doubt that some veterans were very ill.
He said there also seemed to be evidence of a higher rate of illness among troops deployed in Iraq than in other conflicts.
However, he agreed that the stress of military service may be to blame, by weakening the immune system over time.
"A lot more research needs to be done, because the one thing we know for certain is that a lot of good men and women are very, very ill.
"They are not as they were before they went to the war and there is no evidence of gunshot or shrapnel wounds - something else is causing it."