Oral Glycine Side Effects
Oral glycine supplementation is generally well-tolerated at typical doses, but can cause gastrointestinal symptoms (nausea, vomiting, diarrhea) and at high doses (>500 mg/kg) may lead to hypernatremia, osmotic effects, and neurological impairment.
Common Side Effects
The most frequently reported adverse effects from oral glycine are gastrointestinal in nature 1:
- Nausea and vomiting
- Salivation (excessive drooling)
- Lightheadedness
- Diarrhea (particularly with high doses relative to sodium content)
These effects appear dose-dependent and are more pronounced when glycine is given in excess relative to other nutrients 2.
Dose-Dependent Concerns
Osmotic Effects
When glycine is administered in high amounts (particularly >110 mmol/L), it can cause:
- Osmotic diarrhea - excess glycine relative to sodium leads to poor absorption and increased stool volume 2
- Osmotic diuresis - absorbed glycine can increase urine output, potentially causing poor net hydration 2
- Dehydration secondary to fluid losses 1
Electrolyte Disturbances
Significant glycine absorption can trigger 1:
- Hypernatremia (elevated sodium) - documented in 13% of hospitalized infants receiving glycine-based solutions 3
- Hyponatremia (low sodium) - from fluid overload
- Acidosis
- Electrolyte loss
Clinical Pitfall: Hypernatremia developed in some patients, particularly infants, receiving glycine-supplemented solutions, with one case being symptomatic 3. Monitor sodium levels in vulnerable populations.
Metabolic and Systemic Effects
Cardiovascular
- Hypotension (low blood pressure)
- Tachycardia (rapid heart rate)
- Angina-like chest pains 1
Notably, dietary glycine intake has been associated with elevated blood pressure in epidemiological studies, with 2-3 mmHg increases in systolic BP per 0.71g/day increase 4.
Neurological
High-dose glycine (0.8 g/kg) can cause 5:
- Impaired sensorimotor gating (prepulse inhibition deficits)
- Cognitive impairment
- Convulsions (rare, with systemic absorption) 1
- Blurred vision or transient blindness 1
- Vertigo and chills 1
Hepatic and Renal
- Hyperammonemia - can lead to coma and/or encephalopathy 1
- Urinary retention or marked diuresis 1
- Edema (fluid retention) 1
Other Effects
Cytotoxicity Threshold
Critical Safety Concern: Doses exceeding 500 mg/kg body weight may induce cytotoxic effects and contribute to acute glutamate toxicity 6. This represents a clear upper safety limit that should not be exceeded.
Context-Specific Considerations
In Obesity
Paradoxically, while glycine deficiency exists in obesity, supplementation (300 mg/kg for 6 weeks) in obese mice worsened glucose intolerance and increased hepatic gluconeogenesis 7. However, a recent human study (100 mg/kg/day for 2 weeks) showed improved liver markers without adverse metabolic effects 8.
Lack of Therapeutic Advantage
Multiple studies found that glycine-supplemented oral rehydration solutions provided no therapeutic advantage over standard formulations and carried additional risks of hypernatremia 3 and osmotic complications 2.
Allergic Reactions
Allergic reactions to glycine are unknown or exceedingly rare 1, making true hypersensitivity an unlikely concern.
Monitoring Recommendations
When glycine supplementation is used:
- Monitor serum sodium, particularly in infants and vulnerable populations
- Watch for gastrointestinal symptoms suggesting osmotic effects
- Assess hydration status (urine output, weight changes)
- In high-dose scenarios (>100 mg/kg/day), monitor liver enzymes and ammonia levels
- Limit duration and dose to minimize risk
Bottom Line: While low-to-moderate doses of oral glycine (≤100 mg/kg/day) appear reasonably safe in most adults, higher doses carry significant risks of electrolyte disturbances, osmotic complications, and neurological effects. The evidence does not support routine high-dose glycine supplementation outside of specific deficiency states.