Constipation from Memantine

Constipation is defined as infrequent or difficult bowel movements resulting from slowed intestinal motility. In the context of substance use, it most commonly occurs because certain compounds, particularly opioids, bind to mu-opioid receptors in the gastrointestinal tract, causing the smooth muscles of the intestines to contract less frequently and less powerfully. This reduces the propulsive peristaltic waves that normally move material through the digestive system, leading to harder, drier stool that is difficult and sometimes painful to pass.

The subjective experience of substance-induced constipation ranges from a mild reduction in bowel movement frequency to complete inability to pass stool for days at a time. At lower levels, the person may simply notice that their usual digestive rhythm has slowed and that stools have become firmer than normal. At higher levels, particularly during chronic opioid use, the abdomen may become visibly distended and uncomfortable, with a persistent feeling of fullness, bloating, and pressure in the lower gut that can become genuinely painful.

Constipation is most commonly induced under the influence of opioid compounds such as morphine, heroin, oxycodone, codeine, fentanyl, tramadol, and kratom. It is one of the most universal and predictable side effects of opioid use, occurring in the majority of users. Importantly, tolerance to the constipating effects of opioids develops much more slowly than tolerance to other opioid effects such as euphoria, sedation, and respiratory depression. This means that even long-term opioid users who have developed significant tolerance to the desired effects may continue to experience severe constipation throughout their use.

Beyond opioids, constipation can also be induced by anticholinergic compounds, certain antihistamines, and some antipsychotic medications. The mechanism in these cases typically involves reduced secretion of digestive fluids and decreased intestinal motility through different receptor pathways. Dehydration from stimulant use or other causes can exacerbate the condition, as insufficient water intake leads to harder stools regardless of the underlying cause.

Constipation is often accompanied by other coinciding effects such as nausea, appetite suppression, abdominal discomfort, dehydration, and difficulty urinating. These effects together can significantly reduce quality of life during substance use, and the gastrointestinal discomfort is frequently cited as one of the most bothersome side effects of opioid therapy and recreational opioid use alike.

Harm reduction strategies include maintaining adequate hydration, consuming dietary fiber, using over-the-counter stool softeners or osmotic laxatives when needed, and engaging in physical activity when possible. Some cough syrup preparations containing opioids include a mild laxative specifically to counteract this common side effect. If constipation becomes severe and prolonged, it can potentially lead to serious complications including fecal impaction and bowel obstruction, which may require medical intervention.