Complex sentence: Amoxicillin and amoxicillin-clavulanate are recommended by guidelines as the first-choice drug for bacterial sinusitis, but most sinusitis is caused by viruses, for which amoxicillin and amoxicillin-clavulanate are ineffective.
Split into simpler sentences: Amoxicillin and amoxicillin-clavulanate are recommended by guidelines as the first-choice drug for "bacterial" sinusitis. But most sinusitis is caused by viruses, for which amoxicillin and amoxicillin-clavulanate are ineffective.