Because some medicines are only safe while someone is checking.
Examples where monitoring is not optional:
- Levothyroxine — thyroid function, since both under- and over-treatment cause harm
- ACE inhibitors, ARBs and diuretics — kidney function and potassium
- Statins — liver enzymes, at intervals
- Metformin — kidney function and B12
- Lithium, methotrexate, amiodarone and DMARDs — close monitoring, and generally not medicines we take on remotely
The interval is not arbitrary. Kidney function changes with age, illness and other medicines, and a dose that was right two years ago can be wrong now.
We would rather arrange the test than issue a prescription that has outrun its monitoring. A service that issues repeats indefinitely without checking is not being convenient, it is skipping the part that keeps you safe.
Where your NHS GP already monitors you, that is usually the better home for a long-term repeat, and it is free. We will say so rather than duplicate it.