I have to go to the doctor.
'Medico' — ME-di-co. Stress on the first syllable. It's a proparoxytone — three syllables, first stressed.
Use for cancelling due to a medical appointment. In Italy, health is taken very seriously and any medical reason generates immediate understanding and concern.
'Dal medico' uses 'da' + article to mean 'to the doctor's (office)'. Italian uses 'da' for going to a person's place — 'dal medico', 'dal dentista', 'dalla nonna'. This is a key structural difference from English.
Ho una visita medica.
I have a medical appointment.
More formal — 'visita medica' is the standard term for a doctor's visit
Devo andare in farmacia urgentemente.
I need to go to the pharmacy urgently.
'Urgentemente' adds seriousness — pharmacies are first port of call in Italy
Ho un esame del sangue di mattina.
I have a blood test in the morning.
Specific — blood tests require fasting so they explain morning unavailability
Italy has a strong culture of health consciousness. Italians visit their 'medico di base' (GP) regularly and take preventive health seriously. Health appointments are never questioned as reasons to cancel.