Recessed medicine cabinets. Surface mounted medicine cabinets
The bathroom medicine cabinet is often placed above a sink or vanity. It comes with a few adjustable shelves to hold mainly toiletries and medications. Depending on available space you can buy either a single or double cabinet. Single cabinets come with a hinged door while double medicine cabinets have either two sliding or two hinged doors. They have, almost without exception, glass mirrors on the front. Medicine cabinets can be mounted on the surface or recessed for a slimmer look. They are either ready-to-install store bought or custom made. If you go for the latter you have a choice of overall size, shelf configuration and door design. I once installed a custom made recessed medicine cabinet that came with actual architectural drawings and detailed instructions for installation. No kidding!

INSTALLATION
The medicine cabinet can be either surface mounted or recessed into the wall.
Recessed cabinets - new construction
There is no daubt that recessed medicine cabinets look better than surface-mounted ones. They appear to be slim and the door looks like a regular mirror. The best time to install a recessed medicine cabinet is at the time of constructing the house or during major bathroom renovations. If a cabinet light is required wiring can be fed to the unit.
Recessed cabinets - in existing wall
Cutting an opening in an existing wall can be very dangerous and complicated because you do not know what is inside the wall. There could be heating ducts, electrical wiring or plumbing pipes. Also you would have to find out if the wall is load-bearing or not.
Surface mounted cabinets
To avoid a lot of headaches I would recommend installing a surface mounted cabinet. Installation is easy and quick. Just make sure that the mounting screws are long enough to extend at least 16 mm (3/4") into a solid framing member such as a wall stud.
Height off the floor.
There is no golden rule. A medicine cabinet should be high enough off the floor that a small child can't reach it, while being at a convenient height for an adult. The one in my house measures 1.22 m. (4 feet) from the floor to the bottom of the cabinet. If you are a person of average height, you may want to install the centre of the cabinet at eye level.

WARNING
Medicine cabinets do not have locks so make sure that small children cannot get into them. Keep children's step stools out of sight or behind locked vanity doors.