The contact lenses are worn during the day and removed each night before sleeping. They give clear vision for both objects far away and nearby.
The lenses slow myopia progression a bit, although the effect is not strong. The method by which they obtain this control has not been determined. In the best study done to date (CLAMP), the lenses slowed myopia the first year by 34%, but only 12% per year after that.
The greatest risk is that it won't work for myopia control. In the CLAMP study, the average RGP patient went from a prescription of -2.30 to -3.86 over a three year period. RGP day time wear patients also have a greater history of stopping lens wear due to sensitivity to the lenses. The lenses do not fit as snugly as a soft lens and so are more prone to dust getting under the edge of the lenses during wear, creating temporary discomfort. Because of this, they do not work as well in dusty environments. There is an adaptation period of about a week to get used to the lenses.
RGP lenses are what many patients refer to as "hard" contact lenses, although doctors differentiate them from the original "hard" contacts. Today they are made of a material that allows oxygen to flow through the lenses, allowing for greater corneal health. The materials are thus said to be "gas permeable". They are usually smaller than the typical soft contact lens and while they are flexible, they will break if bent too far. The lenses often last for several years and have a history of low adverse events such as contact lens related infections.
RGP lenses are often prescribed due to their greater clarity than soft contact lenses and their ability to fit more complicated prescriptions. The care of the lenses is easier than soft lenses and the yearly cost of upkeep is usually less since the lenses generally last for several years.