As a new campaign is launched to determine the scale of drug rape, BBC News Online looks at some of the issues involved in tackling the crime.
Many people don't want to report the rape until they remember more
Imagine waking up after a night out, knowing something terrible had happened to you, but not being able to recall any of the details.
You didn't think you had had too much to drink, but you have drawn a blank. You know you have been sexually attacked, but you don't know by whom, where, how, or when.
Would you go to the police?
According to Hampshire Police detective inspector Sara Glen, of the major crime department, many people don't report the crime in the first place.
Those who do often leave it too late for the drug to be detected in their system.
"The challenge we face has two factors. One is memory, whether the victim can recall what has happened.
"The typical scenario is that they have been out for the night, something is put in their drink and that is their last memory.
"They wake up in a strange environment, or in their own place, knowing something horrendous has happened.
Prevention not cure
"For the police the problem is knowing what that person really remembers. It can be difficult for us to look into who they were with, where they were and so on."
The amount of memory loss depends on the type and amount of drug given, she said, and sometimes it never returns.
"The other challenge is when people have been drug raped and leave three or four days or more before reporting it. A lot of the evidence is lost by then."
Officers need to know as soon as possible after the attack, so urine samples can be taken at the earliest opportunity.
But victims will often wait until they can remember more details, or until they have a chance to speak to friends about what happened, she told BBC News Online.
"Even if they come and say they think something has happened but they don't know what, it's better than waiting."
Like many forces, Hampshire puts a huge emphasis on prevention rather than cure, with campaigns to warn people about drink spiking.
But the head of a helpline for drug rape victims says the police are not doing enough.
Graham Rhodes, chief executive of the Roofie Foundation, said the police too often "pat people on the head and tell them they were drunk" when they try to report a drug rape.
The charity also campaigns for the Crown Prosecution Service to rethink its definition of an "unreliable witness", because too many cases are rejected when the victim can't remember what happened.
But Mr Rhodes agrees one major problem is that most victims don't get to the police quickly enough. Anything beyond 48 hours is too late for the common rape drugs, he says.
And the lack of specifically trained drug rape counsellors makes it all the more difficult to help people who have left it too late to have the rapist prosecuted, he said.
"Victims often feel guilty as hell, because these drugs sex people up. In their eyes they have seduced their own rapist.
"Sometimes they can remember taking their own clothes off, acting suggestively. In the end they believe they have contributed to their own rape."