Cephalexin and Rimadyl are both known to cause GI upsets, which may be part of the reason that you have witnessed vomiting and he's not willing to eat.
If there are more than 2 hours before the next dosing of the Cephalexin and Rimadyl, I would recommend giving a dose of famotidine (pepcid) to help reduce the discomfort in his stomach. You will want to give 0.5mg/pound of body weight (a 10# ***** would receive 5mg, a 5# ***** would receive 2.5mg, etc). For this, you can visit any human pharmacy and buy the OTC brand name Pepcid, or you can use the cheaper, off-brand “famotidine” that’s available. Either will be useful.
2 hours following a dose of famotidine, the time needed for the medication to begin working, you can offer a bland diet. To make this, you’ll combine white or brown rice, boneless, skinless chicken breast and sufficient water for cooking in a stock pot. Boil on medium until it turns to mush and the breast is easily flaked. To avoid more nausea, start with small amounts to begin with and offer the amount every 2-4 hours. A few teaspoons to start is typically sufficient and you can work your way up every 2-4 hours in incremental increases until you’re sure no vomiting will be seen. If your companion requires a more palatable food, try adding in pureed baby food in chicken, turkey and similar flavors. Avoid those that contain onion or garlic in the ingredient panel.
Try to ensure when you are giving medication that there is at least a small amount of food that goes with it so that it does not further upset his stomach.
If the vomiting/refusal to eat continues, I would recommend having him examined and a determination made if he needs to continue the medication regimen any further due to the symptoms he's developed. Do not be surprised if you see soft stool or diarrhea, as well.
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