A Zanzibari man crouches in a half-built roofless building, struggling to find a vein in his arm, while his friend takes over and injects the heroin for him, drawing blood back into the syringe.

The two are among an estimated 4,000-6,000 narcotics addicts who use syringes to inject themselves in Zanzibar, a tropical archipelago of one million people, better known for tourism and beach holidays than drug abuse.

High rates of HIV among addicts threaten to affect the general population as growth in heroin trafficking through east Africa is making the narcotic more available.

The problem is the increase in (drug) use. There is not any family that hasn't been affected by someone taking heroin, Mahmoud Mussa, coordinator of substance abuse and rehabilitation at the Ministry of Health and Social Affairs, told Reuters.

There is little reliable data on heroin usage. The U.N. Office on Drugs and Crime (UNODC) estimates heroin use in east Africa at between 100,000 and 1.33 million people, twice the proportion of the population using throughout Africa as a whole.

Zanzibar has been established as a major heroin-consuming island, Reychad Abdool, regional human immunodeficiency virus (HIV) advisor for UNODC, told Reuters.

We believe there is an increase in trafficking through east Africa regarding heroin and this is going to be a major threat to building development and security in the future, said Abdool, noting drug use was widely under-reported.

UNODC says east Africa is increasingly becoming a drug transit route on the way from Afghanistan -- which produces most of the world's heroin -- to Europe.

The problem is we have more than 250 unofficial ports, so it's not easy to chase, said Mussa, saying smugglers arrive by sea in traditional dhows, stowing heroin amid ice transported from mainland Tanzania.

HIV RATES HIGH

Measured out on long little finger nails and sold through anonymous windows in Stone Town's winding 19th century streets, the white powder costs 1,000 Tanzania shilling ($0.75) a kete.

The kete is a tin foil wrap with enough for a single hit, about a twentieth of a gram.

Estimates suggest Zanzibar's narcotics addicts, several of whom fill the tourists' favorite haunt of Forodhani Gardens with vacant nodding stares, spend upwards of $10 million a year on heroin fixes. Some spend $30 a day to shoot up several times.

Dozens of discarded needles, many of which are shared by addicts, mingle with rubbish, weeds and tin foil wraps in dumps.

The most affected group by HIV is the injecting drug users because they share the syringe within the group and they sell their bodies to buy drugs, said Suleiman Mohammed Mauly, outreach worker at the U.S.-funded International Center for AIDS Care and Treatment Programs (ICAP).

Officials and ICAP estimates suggest HIV-prevalence on the island is less than one percent, but 26 percent among injecting drug users, whose infections may reach the rest of the general population via a chain of needle-sharing and sex.

Some of the initiatives put in place to tackle drug abuse in Zanzibar include a center to help people quit heroin and get their lives on track. Others are drop-in centres, HIV testing programs and telephone helplines.

I lost my job, I lost my wife, I was chased from home, I was violent and I took anything I could find to exchange for money, said Massoud Mohamed Aboud, 30, who used drugs for nine years but gave up eight days ago.

Afghanistan Medicine