Under previous law, possession of any amount of drugs was punishable by stiff jail sentences, but there was leeway for addicts caught with smaller amounts. In practice, nobody was prosecuted and sentenced to jail for small-time possession, said Bernardo Espino del Castillo, the coordinator of state offices for the attorney general's office.
"We couldn't charge somebody who was in possession of a dose of a drug, there was no way ... because the person would claim they were an addict," he added.
In the past, police sometimes hauled suspects to police stations and demanded bribes, threatening long jail sentences if people did not pay.
"This is not legalization, this is regulating the issue and giving citizens greater legal certainty ... for a practice that was already in place," Espino del Castillo said.
The maximum amount of marijuana considered to be for "personal use" under the new law is 5 grams — the equivalent of about four joints. The limit is a half gram for cocaine, the equivalent of about 4 "lines." For other drugs, the limits are 50 milligrams of heroin, 40 milligrams for methamphetamine and 0.015 milligrams for LSD.
Mexico has emphasized the need to differentiate drug addicts and casual users from the violent traffickers whose turf battles have contributed to the deaths of more than 11,000 people during Calderon's term. In the face of growing domestic drug use, Mexico has increased its focus on prevention and drug treatment.
Sen. Pablo Gomez of the leftist Democratic Revolution Party praised the legislation: "This law achieves the decriminalization of drugs, and in exchange offers government recovery treatment for addicts."