In her grey hooded jacket, gold hooped earrings and dyed black hair, grandmother Sandra stands out among the young emaciated heroin addicts in central Dublin.

Across the road from the Abbey theatre, a group of young men knock back brown medicine bottles of "phi", a mix of liquid methadone.

One of them comes forward hobbling on a crutch. He is desperate for the unlicensed highly powerful painkillers and sleeping tablets which help addicts sleep and numb pain especially during heroin "droughts". Money is exchanged and Sandra hands over a pack of tablets.

A growing army of female traders is dealing in the tablets which are smuggled in from Pakistan and China via Europe. Some are re-packaged in countries such as Croatia and Germany. They cost about €20 (£17.50) for 10 tablets on Dublin's black market.

With 9,500 registered heroin users across Ireland, drugs experts warn the problem is likely to grow, fuelled by a recession that has led many otherwise ordinary people to become dealers.

"Do you think I want to be standing here every day selling these tablets to the addicts?" asks Sandra. "I have never taken drugs in my life and I never thought I would ever be doing this."

But she has debts. Her son Sean died of an overdose and she is still paying the cost of his funeral. "I can get €200 a week selling the 'zimmos' and that is far more than I can get on the social welfare. There is going to be a whole lot more women like me doing this ... it's going to get worse before it gets better," she says.

There are estimated to be hundreds of dealers in central Dublin, earning up to €500 a week. According to campaigners helping addicts, it is women who are the backbone of the business.

Derek Butler produces a paper pharmacy bag containing a phial of prescription sleeping tablets and explains why couriers such as Sandra are doing a brisk trade. "The doctor will only prescribe this amount for the week but I will go through this bag in one single night. I am HIV positive and have three ulcers on my leg. I can't sleep, especially if there is no heroin about. These tabs help me get through the week." Butler says the drugs leave him comatose for up to 15 hours if he takes enough.

The female couriers are a mix of addicts needing to fund their habit and women in debt to loan sharks or simply seeking extra money for Christmas. Mandy, who has been on heroin for 20 years, says that the tablets are not technically illegal but sellers can still be arrested and prosecuted for not having a trading licence.

The Merchant's Quay Project, a charity that works with addicts and the homeless, says it deals with about 9,500 heroin users in 11 of the republic's counties. Tony Geoghegan, its chief executive, says staff have noticed a vast increase in the sale of the smuggled tablets. The project wants to have the smuggled drugs tested because it fears the tablets may be doing as much harm as illegal narcotics.

The head of a busy treatment centre for drug addicts gives two reasons for the emergence of this latest trade. "The recession is pushing people who would have little or no dealings with addicts into this business. It is poverty, pure and simple. And the market is growing because of the drought in heroin," he says.