Extremely bad reaction to methadone

Hi Vets. My cat had acute vomiting on Friday morning, unable to keep any food down and yowling in pain when his abdomen was palpated. We took him to the vets, they ran blood tests and took xrays to check for foreign bodies which came back clear, they said he just appeared to have a lot of gas. His blood tests showed raised ALT levels but everything else normal, I have copies of the blood tests which I will attach. He was given Synthadol (methadone) for pain relief, an anti emetic and sedation/reversal so he could be handled for xrays as he was quite lively. He was given fluids for dehydration. We collected him Friday 5pm, they said the vomiting was likely caused by a tummy bug that would pass with time. They gave him the all clear and sent him home to recover.   As soon as we got him home he vomited a foul rotten egg smelling bile when he got out of the carrier. We called the vets as they said he hadn't vomited the whole time he was admitted. They said it could be due to car sickness on top of the medication but to keep an eye on him and call them in the morning to update. He was drooling a lot, digging with his paws in the water bowl and hanging his head over the water but unable to drink. He hasn't vomited again since but seemed very disoriented and lethargic, this continued throughout the night on Friday.   On Saturday morning he was still very out of it so the vets asked to see him again. They said he appeared to not be in pain, wasn't dehydrated, temperature and other vitals were good but that his lethargy and drooling could be caused by a bad reaction to the methadone.   It's Sunday now and has been over 48 hours since he has eaten anything. He has managed to lap up a small amount of water. He has showed no interest in his favourite foods including boiled chicken, tuna in spring water, dreamies or lick e lix. I have managed to syringe feed him 12ml of Liquivite and I'm waiting to see how that settles, hoping that some nutrition in him will perk him up. He has been to the toilet twice for a pee which appeared to be dark yellow.   So my actual question is: have any vets seen this much of an extreme reaction to methadone before? Could there still be some of the drugs in his system more than 24 hours after he has been discharged from the vets?

Hello!  Let me recap: you took a cat in for vomiting, treatment was given, things appeared to improve with fluids / rest, you took the cat home and they vomited!  This has happened to me as a clinician too, several times,  and can be very frustrating.   Sometimes I wonder if the journey home does it, or if they are simply loathe to vomit in a vets'. I cannot look at your cats blood results - your own clinicians are responsible for that - but I have looked through the BSAVA data-sheets which reckon that the effects of Methodone last for 4 hours but can be for much longer in some individuals, for example if the liver is impaired.     In general I cannot find vomiting listed as a side effect of Methadone.  However your vets' experience of the situation may differ.  Occasionally stressed cats can vomit, which might be the case with an injection. Be that as it may, your cat is continuing to vomit and therefore you need to be aware that they could become dehydrated.  Do they need to be seen by the emergency vet?  Can the emergency team triage the case for you? It may be worth asking either the emergency staff or your own vets what possible causes of vomiting have now been ruled out and what possible causes are still on the differentials list.  Pancreatitis is a common cause of vomiting and abdominal pain so it may be worth asking whether that has been fully ruled out, for example. Best of luck getting to the bottom of this frustrating case.