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LIKELY EFFECTIVE

• Constipation. Rectal glycerol is beneficial in children and adults with constipation. Details: Glycerol rectal suppositories and enemas are approved by the US Food and Drug Administration (FDA) for over-the-counter use to treat occasional constipation in adults and children at least 2 years of age (15).

POSSIBLY EFFECTIVE

• Athletic performance. Oral glycerol may improve hydration during exercise, but clinical studies evaluating glycerol for improving measures of athletic performance are limited and conflicting. Details: Several clinical studies show that glycerol increases serum osmolality and improves hydration status during exercise (2475,2476,93757,93760,93761,93765). One meta-analysis, including 14 studies and 99 patients, shows that glycerol-induced hyperhydration increases body water by approximately 2.5-fold when compared with water-induced hyperhydration (93757). Also, taking glycerol along with table salt 7.5 grams per liter fluid seems to improve hydration status when compared withg glycerol alone (99162). Research regarding the effects of glycerol on thirst, comfort, temperature regulation, sweating, and heart rate is conflicting (2475,2476,93760,99162). While glycerol may improve hydration status during exercise, it is unclear whether it can improve measures of athletic performance. An early meta-analysis including a total of 36 athletes shows that taking glycerol increases endurance performance by 2.6%. Although this is a small difference, it might be considered meaningful to athletes (93757). Some small clinical studies in athletes suggest that glycerol increases endurance and time to exhaustion, with no effect on heat regulation (2475,2479,93764). However, other small studies suggest that glycerol does not reduce perceived exhaustion or improve exercise performance (2476,93760). Also, glycerol has not been shown to increase time to exhaustion in triathletes (2492). When used to improve athletic performance, glycerol is most often taken at doses of 1-1.5 gram/kg in conjunction with approximately 1.5 L of fluid, 60-120 minutes before competition, and in some cases continued during the competition (2475,93757,93760,93764).
• Ichthyosis. Topical application of a specific product containing glycerol and paraffin may improve symptoms of ichthyosis in children. The benefits of topical glycerol alone in these patients is unknown. Details: A large clinical trial in children aged 1 month to 18 years shows that applying a specific product (Dexeryl, Pierre Fabre Laboratoires) containing glycerol 15% and paraffin 10% for 4 weeks reduces symptoms of non-bullous ichthyosis by at least 50% in an additional 17% of children when compared with a placebo cream. Applying this specific product also improves pruritus by an additional 45% when compared with placebo (93756). It is unclear if these findings are due to glycerol, paraffin, or the combination.

POSSIBLY INEFFECTIVE

• Meningitis. Some clinical research suggests that oral glycerol does not reduce the risk of mortality or seizures in adults and children with bacterial meningitis, although it may reduce certain sequelae. Details: A meta-analysis of five low-quality clinical trials including 1,270 patients shows that adjunctive treatment with glycerol as an oral osmotic agent does not reduce the risk of mortality, seizures, or gastrointestinal symptoms in adults and children with acute bacterial meningitis, although it may modestly reduce the risk of deafness and residual neurological deficit in surviving children (99160). Clinical research included in the meta-analysis has used glycerol 1.5 grams/kg every 6 hours in children aged 2 months to 16 years (93762,93763).
• Prematurity. Rectal glycerol does not appear to reduce the time to enteral feeding in premature infants. Details: Glycerol is used as an enema or given by suppository to facilitate the evacuation of meconium in premature infants in order to reduce the time to enteral feeding. However, two meta-analyses of small studies in preterm infants, that include some of the same studies, show that administering glycerol suppositories does not reduce the number of days to full enteral feeds when compared with placebo or no treatment (99166,112003). However, the interpretation of these findings is limited by the heterogeneity of dosages used in the included studies.

LIKELY INEFFECTIVE

• Stroke. Intravenous glycerol may not improve mortality or symptoms in patients with acute ischemic stroke. Details: Several clinical studies show that intravenous administration of glycerol does not improve symptoms of acute stroke (2480,2481,2482,2484,2486). Additionally, one clinical study in elderly patients with acute ischemic stroke shows that while glycerol can improve initial survival, it does not reduce the risk of overall mortality when compared with placebo (2483).

INSUFFICIENT RELIABLE EVIDENCE to RATE

• Dandruff. Topical glycerol has only been evaluated in combination with other ingredients; its effect when used alone is unclear. Details: One clinical trial in adults with dandruff shows that applying a leave-in hair lotion containing glycerol 10%, stearic acid 2.5%, and sunflower seed oil 0.6%, 3 times weekly for 8 weeks, modestly reduces dandruff severity throughout use and for up to one week after the last application when compared with a placebo lotion. The glycerol lotion also reduced total water loss from the scalp by 8% and increased moisturization by 23% (93758). It is unclear if these findings are due to glycerol, other ingredients, or the combination.
• Dry skin. Topical glycerol has only been evaluated in combination with other ingredients for the management of dry skin associated with kidney failure; its effect when used alone is unclear. Details: One small, open-label clinical study in adults with kidney failure and moderate to severe uremic xerosis shows that applying an emulsion containing glycerol 15% and paraffin 10% twice daily for 7 days reduces the severity of xerosis when compared with a placebo emulsion. The glycerol-containing emulsion produces a treatment response in an additional 29% of patients, improves quality of life, and reduces scale density and thickness when compared with placebo (93754). It is unclear if these findings are due to glycerol, paraffin, or the combination.
• Glaucoma. Although there has been interest in using oral glycerol for glaucoma, there is insufficient reliable information about the clinical effects of glycerol for this purpose.
• Intracranial hypertension. Although there has been interest in using intravenous glycerol to reduce intracranial pressure in patients with brain tumors, central nervous system (CNS) trauma, encephalitis, idiopathic intracranial hypertension, meningitis, and Reye syndrome, there is insufficient reliable information about the clinical effects of glycerol for this purpose.
• Meniere disease. It is unclear if intravenous glycerol is beneficial in patients with Meniere disease. Details: A small, uncontrolled clinical study in adults with unilateral, intractable Meniere disease shows that intravenous administration of glycerol 10% at a dose of 0.5 grams/kg once daily for 2 consecutive days, twice monthly for 6 months, reduces the frequency of vertigo attacks and improves quality of life when compared to baseline. Patients reported an average of 3.2 vertigo attacks per month at baseline and 1.2 vertigo attacks per month after 6 months of treatment (106649). The lack of a control group limits the validity of these findings.
• Neonatal jaundice. It is unclear if rectal glycerol improves jaundice-related outcomes in premature infants. Details: A small clinical study in neonates receiving phototherapy for jaundice shows that administering glycerol suppositories has no effect on the duration of phototherapy needed for jaundice treatment, or on bilirubin levels or the rate of bilirubin decline when compared with no glycerol suppositories (99165).
• Obesity. It is unclear if oral glycerol is beneficial in patients who are overweight or obese. Details: One small clinical study in adults on a low-calorie diet shows that taking glycerol 7.5 grams before meals for 6 weeks does not increase weight loss when compared with placebo (2485).
• Otitis externa. Topical glycerol has only been evaluated in combination with other ingredients; its effect when used alone is unclear. Details: One small clinical study in adults with otitis externa shows that inserting ribbon gauze soaked in glycerol and ichthammol into the ear canal reduces pain and swelling as effectively as the application of antibiotic and steroid drops three times daily (93759). It is unclear if these findings are due to glycerol, ichthammol, or the combination. More evidence is needed to rate glycerol for these uses.

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LIKELY SAFE ...when used rectally and appropriately. Glycerol rectal suppositories and enemas are approved by the US Food and Drug Administration (FDA) for over-the-counter use to treat occasional constipation (15,272). ...when used topically and appropriately as a lotion, emulsion, or humectant (15,272,93754,93758,93759,99164).

POSSIBLY SAFE ...when used orally, short-term. Glycerol has been used with apparent safety in clinical trials at doses of up to 1.5 grams/kg (2474,2475,99162).

POSSIBLY UNSAFE ...when used intravenously. While some research suggests that intravenous glycerol can be safely administered for two consecutive days twice monthly for up to 6 months (106649), in another study, hemolysis was reported in 98% of patients treated with intravenous glycerol for acute ischemic stroke (2482).

CHILDREN: LIKELY SAFE
when used rectally and appropriately. Glycerol rectal suppositories and enemas are approved by the US FDA for over-the-counter use to treat occasional constipation in children 2 years of age and older (15,272). ...when used topically and appropriately as an emulsion or humectant in children 1 month of age and older (15,272,93756).

CHILDREN: POSSIBLY SAFE
when used orally, short-term. Glycerol has been used with apparent safety in clinical trials in children 2 months to 16 years of age at doses of 1.5 gram/kg, up to a maximum dose of 25 grams, taken every 6 hours (93762,93763).

PREGNANCY AND LACTATION:
Insufficient reliable information available; avoid using.

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General ...Orally, rectally, and topically, glycerol seems to be well tolerated. Intravenously, glycerol may be unsafe.
Most Common Adverse Effects:
Orally: Bloating, diarrhea, nausea, vomiting, dizziness, and headache.
Topically: Burning, irritation, and pruritus.
Intravenously: Hemolysis in patients with acute ischemic stroke.

Dermatologic ...Topically, glycerol has been reported to cause burning, irritation, and pruritus (93754,93756). Rectally, the regular administration of glycerol 50% enemas has been reported to cause generalized urticaria in at least two patients; in both patients, symptoms resolved after discontinuation (110019,110025).

Gastrointestinal ...Orally, glycerol can cause bloating, nausea, vomiting, thirst, and diarrhea (15,2475).

Hematologic ...Intravenously, glycerol has been reported to caused hemolysis in people treated for acute ischemic stroke (2480,2482).

Neurologic/CNS ...Orally, glycerol can cause mild headache and dizziness (15,2475).

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