By Jennifer Quinn BBC News Online Magazine

The regimen is a month long

The advertisements were splashed all over San Francisco. There were flyers and posters on buses, all making people aware of something called "post-exposure prophylaxis", or PEP.

They warned people that they had 72 hours to start taking the drugs after exposure to HIV, in an attempt to prevent infection with the virus.

It was all part of that city's effort to promote what has been called the HIV "morning-after" treatment. Now, more than five years after the Californian city began promoting the drugs' use, there will be a similar effort to make people in the UK aware of their potential.

The campaign is set to begin at the end of July and will focus on the gay community in London and Brighton.

"It will be raising their awareness of what PEP is and in what circumstances it will be available, and then where they can get hold of it," says Will Nutland, head of gay men's health promotion for the Terrence Higgins Trust.

The campaign comes as the British Association of Sexual Health and HIV's Clinical Effectiveness Group works on a set of guidelines for the use of the drugs.

72-hour window

Currently in Britain, PEP is usually offered to people who might have been infected with HIV through work, such as health care professionals. It's also available at some sexual health clinics, but anyone walking into a casualty unit might be told it's not available there.

PEP is best taken as soon as possible after exposure to HIV, but it can be used up to 72 hours later.

You might get through the month of those drugs and then find out that they didn't work Will Nutland Terrence Higgins Trust

It's sometimes referred to as a "morning-after" option - a reference to the pills women can take to prevent unwanted pregnancies.

But the description is not completely accurate. PEP involves a month-long treatment of powerful anti-viral drugs, and there can be serious side effects.

"It's not a morning-after pill. It's not one pill that you take the next day," Mr Nutland said. "The pills have huge side effects.

"You have to consider that you might have to take time off work. You might have to consider who you would tell. You might get halfway through and decide that you can't stand the drugs.

"And you might get through the month of those drugs and then find out that they didn't work."

The proposed guidelines for PEP's use are likely to be completed by the autumn, says Dr David Hawkins, a physician who is part of the committee.

A draft of the guidelines points out there have been no "randomised studies" to determine how effective PEP is after sexual exposure. But they also say clinical use has shown them to be "protective".

Last resort

In the UK, there are already guidelines in place for administering the drugs after an exposure in the workplace, Dr Hawkins says, and the latest version does mention the possibility of using the therapy after sexual exposure.

But he and other experts say it's not a panacea for the global Aids epidemic. In many cases, people who have been exposed to HIV through sexual contact aren't aware they're at risk for contracting the virus.

"It is not going to make a major impact on the HIV epidemic," Dr Hawkins says. "For those who are at risk, then it's an important thing to be available, to be discussed with clinicians, and to be availed of if it's important."

PEP will increase the control people have over their own health." Ford Hickson Sigma researcher

The guidelines say the decision of whether or not to make the drugs available should be considered on a case-by-case basis.

That's something the Terence Higgins Trust agrees with. The charity intends to have an on-line self-assessment to assist people in determining if they would qualify.

Telephone counsellors will also be available, Mr Nutland says.

"It's clearly important that people know what is available if they've taken a significant risk," Dr Hawkins, who is also a consultant physician at Chelsea and Westminster hospital, says.

Repeat users unlikely

Part of the reason the trust is launching the campaign is to ensure that PEP is available to people other than those who might have been exposed at work.

"It's absolutely an equality issue," Mr Nutland says. Like Dr Hawkins, he believes it won't drive down HIV rates, but could be very important to individuals who have been exposed to the disease.

The Terrence Higgins Trust says huge numbers of people are not expected to come forward demanding the drug, and that not everyone would qualify.

And the report says the possibility of people repeatedly using PEP is unlikely and that those who want to could be encouraged to discuss the issue with a psychologist.

Nevertheless, the treatment is seen by many as a necessary step foward in the fight against HIV.

Ford Hickson, a senior research fellow with the University of Portsmouth's Sigma Research unit, says HIV prevention over the past 20 years has taken a singular route - safe sex, and reducing exposure .

He says: "There are other routes we could be exploring".

Mr Hickson says the therapy would be particularly important for people in relationships where they knew one partner was HIV positive, in case of condom failures, for example.