I've been wearing contacts for 20 years (daily ones).
For the medical, I needed a prescription which also stated I've been wearing the lenses "constantly and successfully for over 8 hours for a period of at least one month" so if you aren't currently wearing lenses, then it may take a while before you can get to that stage.
I also have a Medical cert which states I must "carry a spare set of spectacles" - note that it says spectacles so I'm not sure that Backpacker's substitution of other contact lenses is acceptable (though his cert may be worded differently)??
To be honest, if I lost a lens in flight then it is _way_ easier to pop out the other lens and put on glasses (both of which for me are a one handed operation) than to find a very small lens packet, open it without dropping the lens (both hands needed), check the lens isn't inside out, and then try to insert the lens (again both hands and not clean ones, increasing the chance of irritants on the lens)...