Ruth, please review the following and then return to our conversation with further information and additional questions or concerns if you wish. I'm particularly interested in how long Tiger Lily has been constipated, if there are other worrisome symptoms, and what you might have tried already.
Medical therapy includes stool softeners or laxatives (e.g., lactulose, 0.25-0.5 mg/kg every 8-12 hours (prescription drug)); or docusate sodium/dioctyl sulfosuccinate, 50mg/cat every 12-24 hours (over the counter drug); or mineral oil flavored with vegetable oil at 10-25ml/cat per day) and prokinetics (cisapride, 0.1-1mg/kg every 8-12 hours or 2.5-5mg/cat every 8-12 hours) (prescription drug). Lactulose is the most effective stool softener and is given to effect daily to maintain a soft to semiformed stool (usual dose after titration : 1-4ml/cat every 8-12 hours).
Bulk forming laxatives (cellulose, psyllium) will not be effective in cats prone to dehydration or in cats with poor colonic muscle function because their mechanism of action is similar to high-fiber diets (see below). They can, however, be helpful for simple constipation. Psyllium seed (Metamucil) can be added at 1-4 teaspoonsful per meal added to canned food.
Stimulant laxatives (e.g., bisacodyl, castor oil, cascara) should not be used for relieving constipation but are best used as a preventative in cats that still have normal colonic function.
Prokinetic therapy may assist smooth-muscle function in cats with recurrent constipation or obstipation. The most effective drugs are the serotonergic agonists (cisapride, prucalamide) and histamine H2 receptor antagonists (ranitidine, 1-2mg/kg every 8-12 hours). Only ranitidine (Zantac) is available over the counter.
A high-fiber or low-residue diet may be of value. High fiber diets induce colonic contraction when the patient still has a functional colon and is well hydrated. Avoid high-fiver diets in cats that are prone to dehydration (can exacerbate it). Low residue diets are often best in patients with chronic recurrent episodes of obstipation or true megacolon because they reduce the amount of material reaching the colon and make it easier to keep a soft stool.