People in the US who use cannabis for medical use risk prosecution following a ruling by the US Supreme Court.
Several US states allow medical marijuana on doctors' orders
The Supreme Court ruled that laws in at least nine states that currently allow medical marijuana do not protect users from a federal ban on the drug.
However, Justice John Paul Stevens said Congress could change the law to allow the drug's use for medical reasons.
Advocates say cannabis is more effective than conventional forms of pain relief and without side-effects.
The 6-3 ruling by the Supreme Court is a victory for President George W Bush's administration, which was appealing against a ruling it lost in December 2003.
The court has now ruled that the federal law applies to two seriously ill California women, even though the state allows medical use of marijuana.
Angel Raich, who has an inoperable brain tumour and other medical problems, and Diane Monson, who suffers from severe back pain, were recommended marijuana by their doctors.
Ms Monson grows her own marijuana while Ms Raich receives hers free of charge from her carers.
Justice Stevens said the Controlled Substances Act of 1970 was a valid exercise of federal power by the Congress "even as applied to the troubling facts of this case".