Glycerol is used as a peroral treatment of increased intraocular and intracranial pressure due to its osmotic effect despite the potential increase in blood pressure and blood glucose. We examined the effects of peroral glycerol in diabetic patients and healthy individuals on blood pressure, capillary glucose, and plasma osmolarity. On two separate days, 15 diabetic patients ingested glycerol in doses of 855 and 1710 mg/kg body weight in a randomised, unmasked sequence. Five healthy individuals ingested a dose of 1710 mg/kg body weight. Mean arterial blood pressure (MAP), capillary glucose (CG) and plasma osmolarity (pOSM) were monitored for 180 min. At baseline, the MAP was comparable between the groups of healthy individuals and diabetic patients (p = 0.55), CG was marginal different (p = 0.06), and pOSM values were significantly different (p = 0.007). Following glycerol ingestion, a transient, non-significant increase occurred in blood pressure. Maximal DeltaCG was approximately 1 mM irrespective of the dose and presence of diabetes (p > 0.1). The pOSM response was analysed with a kinetic model and found independent of the presence of diabetes (p = 0.6). The maximal fitted DeltapOSM was 12.7 and 25.3 mOsm/l in the group of diabetic patients after the low and high dose, respectively, reflecting a dose-response relationship. Nausea, fatigue and headache were common side effects. In conclusion, peroral glycerol had similar effects on blood glucose, MAP and pOSM in the diabetic patients and healthy individuals. Specific precautions should not be implemented when treating diabetic patients with a single dose up to 1.7 g/kg body weight. A peak increase of 8% in the pOSM within 1 hr can be expected from this dose.