After the DEA accepts the filing of a petition, the agency must request from the HHS Secretary "a scientific and medical evaluation, and his recommendations, as to whether such drug or other substance should be so controlled or removed as a controlled substance." The Secretary's findings on scientific and medical issues are binding on the DEA. The HHS Secretary can even unilaterally legalize cannabis: "If the Secretary recommends that a drug or other substance not be controlled, the Attorney General shall not control the drug or other substance."
The Obama Administration emphasized smoking marijuana was harmful in response to first marijuana related petitions, but there are more ways to ingest marijuana than smoking.
Marijuana needs to be immediately removed from Schedule I of the CSA.