I'm surprised I didn't notice this sooner, but e-prescriptions will only accept 140 characters in their instructions, just like Twitter. If you still must have longer instructions, you must print the prescription and hand it to the patient for it to be manually filled at their pharmacy.
Then again, maybe doctors will start to use some "twitter-like" abbreviations in their instruction fields now, like:
"Chk ur BP b4 taking b/c itz K 2 hold if nl. TIA."
Dude. That'd b fab.