The marijuana sold will come from state-supervised fields.
The law also allows users to grow their own at home, or join cooperative clubsthat farm it.
Many pharmacists have doubted the financial benefits of selling a cost-controlled product.
Some Uruguayan buyers were also reluctant to sign up to a national registry, complaining of the invasion of their privacy and of having to keep to the monthly limits.
The government has now done a deal with 16 pharmacies, but it hopes to sign up more.
Mr Roballo said there would be a public health campaign before the registry was opened.
He also said the government was not close to covering the predicted demand, but it does not believe there will be "an avalanche of users" signing up,