People were taking a combination of presrciption, OTC and herbal drugs
Nearly half of over 65s are taking five or more drugs, and without regular reviews this may be both dangerous and costly to the NHS, pharmacists say.
The Royal Pharmaceutical Society (RPSGB) says not only are lots of older people taking a "cocktail" of drugs, many are not taking them as prescribed.
Over 60% of 500 polled believed they may be suffering side-effects from the drugs, but many did not tell a doctor.
Older people account for about half of the NHS's £4bn drug bill.
Many of these drugs are on repeat prescriptions, the society notes, and could have been prescribed for conditions the patient no longer has.
The RPSGB polled 457 people aged 65 and above on their medications - which included prescriptions, drugs bought over the counter and herbal medications.
The society is urging all older people to request a medication review from their pharmacist. Regular monitoring of drugs regimens was a recommendation of the 2001 national service framework for older people, with the aim of maximising the benefits and minimising harm.
According to the RPSGB research one in five are not taking medications as prescribed, and one in seven do not take their pills at the recommended time.
The medication of older people has long been an area of concern: the combination of ageing bodies less able to cope with drugs, treatment by multiple doctors for multiple conditions, and known problems in adhering to the instructions, all raise the risk of adverse reactions.
Statistics show that the over-65s are three times more likely to be admitted to hospital because of such reactions, which can lead to falls, delirium and gastrointestinal bleeds among others.
As many as 17% of hospital admission are due to these adverse reactions, and it is estimated that over three-quarters of these "are predictable and preventable", the RPSGB says.
Jonathan Silcock, lecturer in pharmacy at the University of Leeds, said research painted a mixed picture as to the effectiveness of regular reviews by pharmacists in reducing hospital admissions, but a well-trained professional could potentially pick up problems.
"Inevitably people do get put on more drugs when they get older because they suffer from more chronic conditions - this is often the right thing to do, but the problem is we don't necessarily know how these drugs are going to work in combination.
"GPs are excellent at prescribing, but they are not always so good at knowing when to stop, and they may not always have the time to conduct a through review. That's where the well-trained pharmacist could have a key role."
Dr Finbarr Martin, consultant in general and geriatric medicine at Guy's & St Thomas' NHS Foundation Trust, said while pharmacists had an important role, "this is something that all primary care health professionals need to engage with.
"Some older people will be fine on a variety of medications - some will not - and there's not been enough research. We have to really look into constructing a model that makes sure the needs of the frail at the primary care level are met and regularly reviewed - involving GPs, pharmacists and community matrons."
William Bytheway of the faculty of health and social care at the Open University suggested people on long-term medication kept a diary to note any side-effects and advice received.
But he added it was not helpful to "blame older people for poor use of medicines nor, for that matter, to blame the prescribing professionals.
"What is required is a better understanding all round of the benefits and risks of medication in later life; an understanding which makes clear distinctions between essential life-sustaining medication and non-essential medicines and pharmaceutical products."
Michelle Mitchell, charity director for Age Concern and Help the Aged said: "When people are buying over-the-counter medicines it is vital they inform the pharmacist of any other prescription drugs and medicines they are taking.
"Getting the correct advice and reassurance could very well be the best medicine of all."