The homelessness charity uses the figure in its 'Give Where it Counts' campaign to increase awareness in Lambeth of its work with health agencies, which helps get people off the streets and rebuild their lives.
Some 80 posters were placed at tube stations in Lambeth last week and 20 more will be sited on London Underground escalator panels next Monday. They were funded by Lambeth Council in partnership with local police and drug-treatment agencies.
"This is our boldest campaign," said David Devoy, St Mungo's head of services. "It follows others that showed how we can stabilise people's lives without mentioning drugs. It could be seen as controversial, but the overriding message is how the public can best help the homeless."
Devoy believes many people are not aware of the extent to which the homeless use drugs.
"We aren't trying to discourage compassion," he said. "But many people think that giving spare change is all they can do, and we want to show them otherwise."
Devoy added that the charity hopes to persuade other London boroughs to fund the campaign.