Possible Clues:

• Rx instruction
• Three times daily, on an Rx
• Prescription abbr.
• Three times daily, in Rx's
• Pharmacist's abbr
• Where seawater remains after an ebb
• Three times a day, in prescriptions
• Three times a day, to an MD
• Thrice daily, on an Rx
• Three times a day, on an Rx