I am not terribly familiar with the world of contact lenses. I wear glasses exclusively. But lots of people who wear contacts also wear glasses part-time. Until recently, it was a frequent practice of eye doctors to refuse to give contact wearers their own prescription after it was measured. This meant that if you went to get fitted for contacts, you would be forced to continue to buy contacts from that eye doctor in the future, because they would not release your prescription. Eye doctors, of course, feared their patients would get mail order contacts in future.
A law passed, titled “Fairness to Contact Lens Consumers Act” that requires your eye doctor to provide your contact lens prescription after fitting you for contacts.
How does this apply to glasses? You can use your prescription for your contacts to fill a prescription for glasses. Therefore, you can use your contact prescription to order glasses online. Note, however, that you can NOT use a glasses prescription to fill a contact prescription. This is because a contact prescription has additional information about the shape of the eye.
As an additional note, your eye doctor also cannot refuse to provide your prescription for glasses, but this law has been around since the 1970s. So, if you have been fitted for glasses by a particular optometrist, your doctor is legally obliged to provide your prescription upon request. Don’t let an optometrist say, “we can’t/don’t provide that information.” You have a legal right to your own prescription.
Over at The Consumerist, a blogger provides a few more details.