Patients prescribed anti-psychotic drugs could be the least likely to pick up their medicines, figures suggest.
Data from the NHS Information Centre for England shows a higher percentage of anti-psychotic prescriptions go unredeemed compared to other drugs.
Some 85.1% anti-psychotics are actually dispensed to patients, compared to 98.5% of other drugs.
The figures, from 145 GP practices, cover the period 2004 to 2008 and involve a comparison of prescriptions written by GPs and what is actually dispensed by pharmacies.
Overall, the typical pick-up rate for all drugs fell slightly from 99.6% in 2004 to 98.5% in 2008.
One explanation is that improved recording systems in GP practices could be partly behind this difference.
Generally, GP practices with a large proportion of prescriptions for elderly patients had the highest redemption rate while practices with a large proportion of prescriptions for young patients had the lowest.
NHS Information Centre chief executive Tim Straughan said: "The study suggests that in the main, patients are redeeming their prescriptions - but there appears to be a lower redemption rate for anti-psychotic prescriptions.
"A patient may not redeem their prescription for several reasons such as deciding they don't need the medication or a reluctance to take a drug they have had side effects from previously.
"A patient may have lost their prescription or obtained their medication through another source - such as during a stay in hospital.
"The cost of prescriptions may also be a factor - although it is worth noting that another NHS IC report shows that only about one in 20 prescriptions are paid for by the patient."