A SMOG enema is a mixture of saline, mineral oil, and glycerin administered rectally to relieve severe constipation or fecal impaction. The name is an acronym: S for saline, M for mineral oil, O (sometimes grouped with the mineral oil), and G for glycerin. It’s most commonly used in pediatric settings, where children’s hospitals use it as a go-to treatment when oral laxatives haven’t worked or when stool is too hard and compacted to pass on its own.
What’s in a SMOG Enema
The three ingredients are mixed in equal parts, a 1:1:1 ratio. Each component does something different. Normal saline (salt water) adds volume and draws water into the bowel through osmotic pressure. Mineral oil coats and softens hardened stool, making it easier to move. Glycerin acts as a lubricant and mild irritant to the intestinal lining, triggering the muscle contractions that push stool out.
Because SMOG enemas are compounded on-site rather than purchased as a pre-made product, the exact preparation varies by hospital. Boston Children’s Hospital, for example, specifies making saline from 1½ teaspoons of table salt per 1,000 mL of warm tap water, with glycerin doses ranging from 5 to 30 mL depending on the patient. The mixed solution is administered at body temperature to minimize cramping.
When It’s Used
SMOG enemas are primarily used for fecal impaction, a condition where a large, hard mass of stool gets stuck in the rectum or lower colon and can’t be passed naturally. This is different from everyday constipation. Fecal impaction can cause significant pain, bloating, and in some cases nausea or loss of appetite. Children with chronic constipation are especially prone to it.
In hospital protocols, a SMOG enema typically comes into play after a physical exam confirms impaction. UNC School of Medicine’s pediatric protocol, for instance, calls for a SMOG enema dosed at 10 mL per kilogram of body weight. Children’s Hospital Colorado specifies 60 mL for children aged 2 to 11 and 120 mL for those 12 and older, with the option to repeat the dose up to twice per day. It is not recommended for children under 2.
How It Works
Once the solution is introduced into the rectum, it softens and lubricates the impacted stool while the saline draws additional fluid into the bowel. This combination increases pressure inside the lower intestine, which stimulates peristalsis, the wave-like muscle contractions that move material through the digestive tract. The urge to have a bowel movement usually hits within two to five minutes. The goal is for the patient to retain the enema long enough for it to work but not so long that it causes significant discomfort.
SMOG vs. Milk and Molasses Enemas
You may see SMOG enemas mentioned alongside milk and molasses enemas, which serve a similar purpose but use different ingredients. Milk and molasses enemas work through a related mechanism: the sugars in molasses interact with the intestinal lining and produce gas, and the resulting distention and pressure trigger evacuation. However, milk and molasses enemas have fallen out of favor at some institutions. One emergency department used them for over eight years before a regulatory review pulled them from the formulary, citing concerns that the preparation wasn’t sterile and that using milk and molasses this way was considered off-label.
SMOG enemas avoid some of these issues because their ingredients (saline, mineral oil, glycerin) are standard medical supplies rather than food products.
Risks and Side Effects
Enemas of any kind carry some risk. A study of 261 patients receiving enemas for constipation in the emergency department found a complication rate of 3.1%. The most common issues were an elevated heart rate (four cases) and a drop in blood pressure (two cases). One patient experienced increased pain, and one developed a fever afterward. More serious but rare complications include electrolyte imbalances, rectal bleeding, bowel perforation, and infection.
Cramping during and immediately after the enema is common and expected. The sensation of urgency can be intense, particularly for young children who may not understand what’s happening. Most side effects resolve quickly once the bowel empties.
Why It’s Mostly a Pediatric Treatment
SMOG enemas appear far more frequently in children’s hospital protocols than in adult care guidelines. This is partly because chronic functional constipation is extremely common in children, accounting for a significant share of pediatric gastroenterology visits. Kids are also more likely to develop impaction from stool-withholding behavior, where they avoid going to the bathroom due to pain or anxiety, creating a cycle that worsens over time. The SMOG enema offers a relatively quick mechanical solution when the impaction needs to be cleared before longer-term management with diet changes and oral medications can begin.
Adults with fecal impaction are more commonly treated with commercially available fleet enemas or manual disimpaction, though the underlying principle is the same: soften the stool, lubricate the passage, and stimulate the bowel to contract.