We present three cases of rare side effects which appeared to be attributable to antibacterial drug treatment. A 57-year-old female patient was admitted to hospital due to increasing dyspnea. Computed tomography revealed interstitial lung fibrosis which was attributed to the toxic effects of nitrofurantoin (50 mg/d) that the patient used for approximately one year for recurrent urinary tract infection. She died two weeks after hospital admission due to acute respiratory failure. A 20-year-old male patient presented with most intense headache and psychomotor deceleration. Pseudotumor cerebri, which was suspected to be the underlying cause, is described as a rare side effect of minocyclin which the patient has taken for acne pustulosa (100 mg single dose). After dechallenge of minocyclin, neurological symptoms quickly subsided. A 82-year-old female patient used moxifloxacin (400 mg/d) for febrile bronchopulmonary infection for one week. During this therapy, confusion and severe dementia presented and remained for more than two months after discontinuation. The demential syndrome appears to be possibly related to the fluoroquinolone use. In summary, adverse drug effects not pertaining to the primary physician's field are especially difficult to identify. Most importantly, rare side effects must be borne in mind by the prescribing physician.