The supreme court on Monday rejected an effort by Nebraska and Oklahoma to have Colorado’s pot legalisation declared unconstitutional.

The justices did not comment about their dismissal of the lawsuit that the states filed directly against their neighbour.

The court also ordered Massachusetts’ top court to look again at the state’s ban on stun guns.

Nebraska and Oklahoma argued that Colorado’s law allowing recreational marijuana use by adults ran foul of federal anti-drug laws. The states also said that legalised pot in Colorado was spilling across the borders, complicating their own anti-drug efforts and draining state resources.

The Obama administration sided with Colorado, despite its opposition to making marijuana use legal.

Justices Clarence Thomas and Samuel Alito would have heard the states’ lawsuit.

In the Massachusetts case, the justices revived an appeal from a woman who said she kept a stun gun in her purse for self-defense against an abusive former boyfriend.

The Massachusetts supreme judicial court said the ban on possession of stun guns does not violate the second amendment.

Monday’s unsigned US supreme court opinion did not go so far as to strike down the law or say stun guns were protected under the amendment’s “right to keep and bear arms”.

But the opinion said the Massachusetts court’s reasoning for upholding the law was faulty.