In the UK, the Health & Safety Executive has produced a guidance note called “Management of Firearms and Weapons in Film & TV Productions“. This note covers the Health & Safety issues connected with weapons, but there are other legal instruments that should also be consulted, including the Firearms Acts, the Violent Crimes Reduction Act 2006 (VCRA) and the Crime and Security Act 2010. The local police force must be informed if there are any weapons audible or visible to the general public (not in the theatre).
A fight director should always be consulted where any weapons are used in a fight situation.
There are several well-documented instances of actors injuring themselves with weapons.
In 2008, Daniel Hoevals accidentally slit his own throat on stage in Vienna, when a fake knife was swapped for a real one without his knowledge. [News Article]
Retractable knives should not be used on stage, due to the risk of them jamming and not retracting. Any fight using weapons should be supervised by a fight director, who can ensure the choreography is safe.