Patients show up each day to receive their treatment in small doses handed through a small window.
Then they gather around a table to shoot up, part of a pioneering Swiss program to curb drug abuse by providing addicts a clean, safe place to take heroin produced by a government-approved laboratory.
The program has been criticized by the United States and the U.N. narcotics board, which said it would fuel drug abuse. But governments as far away as Australia are beginning or considering their own programs modeled on the system, which is credited with reducing crime and improving the health and daily lives of addicts.
Swiss voters are expected to make the system permanent Sunday in a referendum prompted by a challenge from conservatives.
The heroin program has won wide support within Switzerland since it was begun 14 years ago to eliminate scenes of large groups of drug users shooting up openly in parks that marred Swiss cities in the 1980s and 1990s.
Zullino's office, part of the Geneva University Hospitals, is one of 23 such centers in Switzerland.
Patients among the nearly 1,300 addicts whom other therapies have failed to help take doses carefully measured to satisfy their cravings but not enough to cause a big high. Four at a time inject themselves as a nurse watches.
In a few minutes most get up and leave. Those who have jobs go back to work.
"Heroin prescription is not an end in itself," said Zullino, adding that the 47 addicts who come to his office receive a series of additional treatments, such as therapy with a psychiatrist and counseling by social workers.
"The aim is that the patients learn how to function in society," he said, adding that after two to three years in the program, one-third of the patients start abstinence-programs and one-third change to methadone treatment.
"Thanks to this policy we don't have open drug scenes anymore," said Andreas Kaesermann, a spokesman for the Social Democrat Party, part of the coalition government.
A mid-November survey of 1,209 voters by the respected gfs.bern institute indicate the program will be easily approved, with 63 percent of voters favoring it compared with 21 opposed. The poll had a margin of error of 2.9 percentage points.
Health insurance pays for the bulk of the program, which costs 26 million Swiss francs ($22 million) a year. All residents in Switzerland are required to have health insurance, with the government paying insurance premiums for those who cannot afford it.
"It's wrong that the health insurance pays for this," said Alain Hauert, spokesman for the right-wing Swiss People's Party. He said the state should invest more money into prevention and law enforcement.
Crimes committed by heroin addicts have dropped 60 percent since the program began in 1994, according to the Federal Office of Public Health says.
And, Zullino said, patients reduce consumption of other narcotics once they start the heroin program and suffer less from psychiatric disorders.
But, he added, "the idea has never been to liberalize heroin. It's considered a medicine and used as such."