Hospitals have been told to do more to cut out medication errors after figures showed 40,000 mistakes a year are made.
About 40,000 medication errors were made in the last year
Most errors caused no harm, but 2,000 led to moderate or severe harm, or death, as in 36 cases.
The Healthcare Commission urged the NHS to improve how it prescribed and dispensed drugs as it published ratings for all 173 hospital trusts in England.
The watchdog classed 85 trusts as fair or weak. NHS chiefs said hospitals needed to be honest about the problems.
The medication errors figures, given to the Healthcare Commission by the National Patient Safety Agency NPSA, cover incidents in England and Wales the 12 months to July.
They showed about 80% caused no harm, 15% low harm and 5% moderate or severe harm.
Only 18 trusts in the watchdog's review of medicines management were rated as excellent, while 70 were good, 73 fair and 12 weak.
The Healthcare Commission said more needed to be done to discuss side effects with patients, to give out written information as required by law, and to minimise risks from injected drugs.
Trusts were measured in 21 areas, including whether patients had had a comprehensive medicines review and if they had a complete medicine record for their stay in hospital.
The review did find areas of good performance, including 40% of trusts prudently using antibiotics to help cut MRSA rates.
But the watchdog said there was a need for improvement, including making sure patients understand the purpose and potential side effects of medicines.
The report said pharmacists also needed to spend more time on the wards to minimise errors, with 11 of the 12 trusts that scored weak overall performing poorly in this area.
Other areas needing attention included hospital patients not being given control of their medicines, even though they managed perfectly well at home - 69% of trusts said this was not possible on a fifth of their wards.
Parkinson's disease patients were cited as a group who often preferred managing their medication as timing of dosing is vital.
Commission chief executive Anna Walker said while many trusts were getting the basics right, there was still "some way to go when it comes to involving patients in decisions about medicine".
"Trusts need to do more talking to patients about their medicines and their potential side effects.
"They need to make sure patients feel empowered to discuss any concerns."
Steve Ford, chief executive of the Parkinson's Disease Society, added: "Difficulties could be avoided if ward staff had a better understanding of the condition and of why the timing of Parkinson's drugs is crucial."
Maria Nyberg, policy manager at the NHS Confederation, which represents NHS trusts, said there were some examples of good practice, but the publication was a positive as "identifying weaknesses or problems" helped to tackle them.
"The only way the service will achieve real improvements for patients is by being frank about the problems and challenges that it faces."
And a Department of Health spokeswoman added: "Hospitals are working very hard to ensure that patients are getting the most from their medicines. "There is, however, room for improvement in some areas."
The NPSA said the number of medication errors and deaths should be seen in the context of the 1m people seen by the NHS every day.