Their study, published in the journal JAMA Internal Medicine, argued elderly people had the least to gain.
The team at University College London (UCL) has called on doctors to discuss the risks more clearly with patients.
The charity Diabetes UK said doctors needed to strike a careful balance when prescribing treatment.
Type-2 diabetes is an inability to control blood sugar levels and is linked to lifestyle factors including diet and obesity.
In the long-term it is linked to heart disease, kidney damage, nerve damage and even blindness.
Drugs such as metformin can be used to lower blood sugar levels to prevent the side effects.
The report, which was carried out by the University of Michigan as well as UCL, said an average 45-year-old who dropped their blood sugar levels by one percentage point would gain 10 months of healthy life, compared with three weeks for a 75-year-old starting treatment.
It added this needed to be balanced against the down-sides to a lifetime of medication including: Having to have multiple injections and blood sugar tests each day, drugs causing indigestion or sickness, or insulin treatments risking blood sugar levels dropping dangerously low (hypoglycaemia).
One of the report's authors, Prof John Yudkin, told the BBC: "What it means is if you're someone with type-2, it's your right to know what the benefits of the treatment are in terms of gain in life expectancy or reduction in heart attacks or going blind.
"And then you are entitled to decide, but not many doctors have got those figures to hand."
He said GPs were too "target focussed" and were often looking only at the blood sugar level.
The findings do not apply to people with type-1 diabetes.
Commenting on the report, Simon O'Neill, the director for health intelligence at Diabetes UK, said: "Sometimes there is a balance to be struck where certain medications might help give someone a longer life, but also cause side effects that might negatively impact on quality of life.
"This study highlights the importance of looking at the individual needs of the person with type-2 diabetes, rather than adopting a blanket approach."
Patients are advised not to make any decisions without consulting their GP.
The National Institute for Health and Care Excellence, which advises the NHS on medication, said: "The current NICE guidance on the management of type-2 diabetes recognises that glycaemic [sugar] control targets should not impair a person's quality of life as a result of the side effects of treatment.
"Where medication does not help achieve this target level, lifestyle therapies such as dietary advice should be offered."
Refreshed guidelines are due to be published next summer.

AM Simon Thomas said it would use Wales' share of proceeds from UK ministers' apprenticeship levy, around £150m a year, to fund the plan.
He said it would boost young people's prospects and plug "skill gaps".
On Tuesday, Welsh Labour ministers outlined plans to spend an additional £5m on apprenticeships in 2016-2017.
Mr Thomas launched the proposals with Plaid leader Leanne Wood at a gas training academy in Tredegar, Blaenau Gwent.
Although aimed at young people, Plaid Cymru said it would put no age restrictions on its proposed scheme.
The downsides of taking medication for type-2 diabetes may exceed the benefits for some patients, researchers have advised.
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Plaid Cymru would create 50,000 apprenticeships if it wins power in May's assembly election, the party has announced.