Amoxicillin oral 1g 8 hourly for 5 days

or

Doxycycline oral 200mg stat, then 100mg once daily for 5 days

N.B. In pregnant patients, doxycycline is contraindicated. See here for an alternative option in penicillin allergic pregnant patients.

80% of cases resolve in 14 days without any antimicrobial treatment.

Patients with untreated bacterial disease rarely develop severe infection or complications. 

• Acute rhinosinusitis is a viral infection associated with the common cold and resolves in 7 to 10 days; bacterial superinfection occurs in <2% of episodes.
• Consider antibiotic treatment if a patient with an improving viral upper respiratory tract infection develops new symptoms of sinusitis and a fever, or if symptoms of sinusitis persist for 10 or more days without improvement.
• Suspect orbital involvement if there is peri-orbital oedema, a displaced globe, double vision, ophthalmoplegia, or reduced visual acuity.
• Suspect intracranial involvement or Pott's Puffy Tumour if there is a severe frontal headache, frontal swelling, symptoms or signs of meningitis, or focal neurological signs.

Investigations are not required in primary care. Nasal swabs for culture have poor diagnostic yield.