Psyllium fiber is commonly recommended as part of the management for anal fissures and, to a lesser extent, anal fistulas. The evidence is based on clinical guidelines and small randomized controlled trials. The primary rationale is that psyllium, a bulk-forming laxative, increases stool bulk and moisture, making defecation easier and less traumatic to the anal mucosa. This reduction in stool hardness and straining can facilitate healing of fissures and help prevent recurrence. For anal fissures, several studies and systematic reviews (e.g., Nelson RL et al., 2012, Cochrane Review) have shown that fiber supplementation, including psyllium, can improve healing rates and reduce pain compared to placebo, though the effect size is moderate. Guidelines from gastroenterology societies recommend fiber as first-line therapy for acute anal fissures.

For anal fistulas, direct evidence supporting fiber is limited. Its use is largely extrapolated from its benefits in fissure and general anorectal health—softer stools reduce trauma and may help symptomatically, but do not directly heal fistulas. Thus, while scientific evidence supports fiber for fissures (with moderate-quality data), its use for fistulas is more adjunctive and based on indirect rationale rather than direct proof.

Overall, psyllium fiber is scientifically validated for anal fissure management, with an evidence rating of 2/5 due to the quality and size of available studies. Its role in fistula is much less clear.