Rule 28. Medical units exclusively assigned to medical purposes must be respected and protected in all circumstances. They lose their protection if they are being used, outside their humanitarian function, to commit acts harmful to the enemy.

https://ihl-databases.icrc.org/en/customary-ihl/v1/rule28#:~:text=to medical units-,Rule 28.,and protected in all circumstances.

the protection of medical units ceases when they are being used, outside their humanitarian function, to commit acts harmful to the enemy. This exception is provided for in the First and Fourth Geneva Conventions and in both Additional Protocols.[37] It is contained in numerous military manuals and military orders.[38] It is also supported by other practice.[39]

While the Geneva Conventions and Additional Protocols do not define “acts harmful to the enemy”, they do indicate several types of acts which do not constitute “acts harmful to the enemy”, for example, when the personnel of the unit is armed, when the unit is guarded, when small arms and ammunition taken from the wounded and sick are found in the unit and when wounded and sick combatants or civilians are inside the unit.[40] According to the Commentary on the First Geneva Convention, examples of acts harmful to the enemy include the use of medical units to shelter able-bodied combatants, to store arms or munitions, as a military observation post or as a shield for military action.[41]