Healthy people should not take aspirin to ward off heart attacks and cancer, according to the most comprehensive review of the risks and benefits.
There has been growing debate about whether all people over 50 should take a daily, low dose aspirin.
But the review, conducted by the research arm of the NHS, said it was a "fine balance" due to the dangers of bleeding in the brain and stomach.
Overall it warned against taking the drug, until there was more evidence.
Aspirin makes the blood less sticky so it reduces the odds of a blood clot forming inside the body, which could cause a heart attack or stroke.
There are even studies suggesting it can cut the risk of some cancers.
However, as the drug makes it harder for the blood to clot it can cause problems inside the body.
The drug is given to people at high risk of a heart attack or stroke as the medical benefit is clear.
However, there have been calls to give aspirin to otherwise healthy people as well.
A team at Warwick Medical School was asked to assess the evidence by the NHS National Institute for Health Research.
For heart attacks and strokes, they concluded giving everyone aspirin would cause "net harm due to increased potential for bleeding".
This was in part due to better management of at-risk patients including prescribing drugs to lower blood pressure.
On cancer, they concluded the evidence was not strong enough to base a decision on, but trials taking place would give clearer proof in the next five years.
Prof Aileen Clarke, who led the review, told the BBC: "The risks are finely balanced and for now there is not the evidence to advise people to take it.
"It would be lovely to say over-50s should take an aspirin a day and have much less cancer, but the research hasn't yet been done and we should be cautious.
"We need to be extremely careful about over-promoting aspirin."
Amy Thompson, senior cardiac nurse at the British Heart Foundation, said: "Aspirin is extremely important for many heart patients, but for people free of heart disease the jury is still out as the risks are likely to outweigh the benefits.
"Further research is underway which will shed light on who else is likely to benefit the most from taking aspirin."