About 1,000 detainees are involved each year in incidents that result in serious illness or self-harm while in police custody, a report has suggested.
Detainees have in some cases stolen drugs from forensic officers
The incidents - called "near misses" - include drug and alcohol use, self harm and medical conditions like diabetes.
Forensic medical examiners studied the Met Police for a year, then estimated the overall data for England and Wales.
The report said police action often saved lives, but forces should improve to avoid near misses in future.
An estimated 400 of the 1,000 incidents would probably have resulted in death if someone had not intervened, the report said.
The study, by the Forensic Medical Examiners (FME) and the Met, was published by the Independent Police Complaints Commission (IPCC).
It was commissioned to get a better understanding of what goes wrong, and right, when incidents happen so future near misses and deaths can be prevented.
The overall figure was estimated after researchers found there were 121 near misses between May 2005 and April 2006 in the Metropolitan Police Service (MPS).
Twelve of those - who were mainly using drugs or alcohol - had to be resuscitated in the custody suite. In some cases they had swallowed drugs when they were arrested, to try to hide them from police.
Nearly half of all near misses involved suicide and self-harm, a third involved drugs, some 14% were due to medical conditions and 7% were connected with alcohol.
Researchers said they were surprised "to find three near misses in our sample where detainees had snatched and consumed drugs in the possession of an FME".
There were six deaths in custody in the MPS and 28 deaths in custody in England and Wales during the period of the study.
The Met said the number of deaths in custody had decreased from 12 in 1995/6 to one last year, which it said reflected the positive work being done.
IPCC chairman Nick Hardwick said "more can be done" to reduce the likelihood of deaths and near misses.
"We need to be careful about how we think about near misses. In many cases the actions of custody staff prevent a death and the individual is left unscathed.
"However, in some instances they may be seriously and permanently injured. Not all near misses end happily."
Jason Payne-James, a co-author of the IPCC report, said the way to reduce incidents was through good risk assessment and better healthcare provision.
"Forces need to ensure that their custody staff are adequately trained to identify the healthcare issues they are likely to face on a day to day basis."
In suicide attempts most detainees tried to strangle themselves using items ranging from shoelaces to paper suits issued in custody.
One recommendation called for police forces to make sure custody officers had better access to ligature knives, which can safely cut detainees free.
The recommendations also included making custody staff fully aware of how to respond to drug swallowing and severe intoxication, and ensuring custody officers check cells when they are vacated and remove items which could be used by others to self-harm.
Staffing levels and the amount of checking of detainees were also found to have an effect on the number of near misses.
Mr Hardwick added it was important to realise the Met saw 300,000 detentions a year, "with detainees sometimes deliberately putting themselves at risk".
"The police officer in this role has an unenviable task," he said. "What is the difference between a near miss and an actual death? It's not magic, it's about following basic procedures.
"It's about an experienced custody sergeant, who is prepared to say, 'hang on a minute, this doesn't feel right', and get the medical back-up that's needed."
The Association of Chief Police Officers (Acpo) and the Met welcomed the report.
Earlier this month doctors the British Medical Association said the health and safety of prisoners was being jeopardised by putting them in ill-equipped cells at police stations and courts.