Chemotherapy in cats is not chemotherapy in people. Doses are lower, cats tolerate the drugs better than dogs do, and most side effects get handled at home with a phone call. Two windows decide how a cat does: the gut window on days 3 to 5 after a dose, and the marrow window on days 7 to 10.
Why the dose is lower than a dog's
Cats handle some drugs well and others badly, so protocols are built around that split. Doxorubicin is usually capped near 1 mg/kg in cats, roughly 70 to 80 percent of a common canine dose. Cisplatin is avoided outright because it causes fatal pulmonary edema in cats at doses dogs tolerate. Chlorambucil, the workhorse for small-cell lymphoma, runs 2 mg per cat every other day, or 15 mg/m2 once weekly, which is a small fraction of a human dose.
The payoff is that most cats finish a 4 to 6 month protocol without a dose reduction. Hair loss is rare. Whiskers may thin or grow back curly after 2 to 3 cycles, and a dull coat for 2 to 3 weeks is normal. Large patches of missing fur point to something else. If the diagnosis is still fresh, the staging basics sit in lymphoma in cats.
The gut window: days 3 to 5
Nausea and vomiting from chemotherapy peak 72 to 120 hours after the dose, not on the day of treatment. Owners relax on day 1, then get caught out on day 4. Score four things once a day: interest in food, lip licking, swallowing, and stool form. That takes 30 seconds and catches most trouble early.
Antiemetics work best given before the problem, not after. Maropitant at 1 mg/kg under the skin or by mouth every 24 hours, started the morning of treatment and continued for 4 to 5 days, is a common protocol. It is a prescription drug: the dose and duration come from the vet managing that cat, based on weight, kidney values, and the drug used, and are not something to copy from another cat. Mirtazapine at about 1.88 mg per cat (half of a 3.75 mg tablet) every 48 hours is used when appetite is the main problem.
Diarrhea is usually mild and settles in 2 to 3 days on a bland diet. A probiotic with Enterococcus faecium at 1 capsule daily and 1 to 2 tablespoons of plain canned pumpkin per meal are standard home measures. Vomiting more than 3 times in 24 hours, or any vomiting with lethargy, is a same-day call: a cat on chemo dehydrates in under 24 hours.
The marrow window: days 7 to 10
White cells bottom out 7 to 10 days after a dose and recover by day 14 to 21. That gap is why a cat can look fine on day 6 and run a fever on day 9. Most protocols check a CBC between day 7 and day 10 during the first 2 cycles, and drop to once per cycle after that if counts behave.
Rough thresholds vets work with: hold the next dose if neutrophils fall under 1,500/µL, and treat with antibiotics plus a recheck in 24 hours if they fall under 1,000/µL or the cat has a fever of 39.4 C (103 F) or higher. A temperature taken at home with a digital thermometer is worth more than a guess; 38.1 to 39.2 C is the normal band.
Day-by-day watch schedule
| Day after dose | What peaks | Check at home | Call threshold |
|---|---|---|---|
| Day 1 | Mild sedation from premeds | Appetite, injection site | Temp above 39.4 C |
| Day 3 to 5 | Nausea, vomiting, diarrhea | Food eaten, stool, lip licking | 3 vomits in 24 hours |
| Day 7 to 10 | Neutrophil nadir | Energy, temperature twice daily | Temp 39.4 C or no food for 24 hours |
| Day 14 | Recovery, weight drift | Weight on a baby scale | Loss over 5 percent of body weight |
| Day 21 | Next cycle due | CBC before the dose | Neutrophils under 1,500/µL |
Weigh twice a week on a kitchen scale with the cat in a box, not once a month. Weight loss of 5 percent in 2 weeks is the earliest hard signal that a protocol needs adjusting, and it shows up before the cat looks thin. A cat that keeps eating but still loses weight needs a parasite and thyroid check alongside the cancer review; the pattern is covered in weight loss with a normal appetite.
Quality-of-life scoring beats guesswork
Score 6 items from 0 to 5 each, for a maximum of 30: appetite, drinking, activity, grooming, social contact, and signs of pain. Do it on the same day each week, ideally before the next dose. A single score of 18 out of 30 in a cat having a bad week is not a stop signal. Two consecutive weeks under 12 out of 30, with no reversible cause such as nausea or a dental problem, is the point where most oncologists raise dose reduction, a drug switch, or stopping.
Write the scores down; the trend line is the whole value of the exercise.
When to stop, and what to ask
Common stopping points: disease progression on 2 consecutive imaging checks, grade 4 toxicity (neutrophils under 500/µL or platelets under 50,000/µL), quality-of-life scores under 12 out of 30 twice, or unmanaged vomiting that keeps the cat from eating for 48 hours. None of these are failures. Palliative prednisolone at 1 to 2 mg/kg daily, or 0.5 to 1 mg/kg every 24 hours long term, often buys good weeks with far fewer clinic visits.
Keep the vaccine and deworming schedule separate from chemo weeks; live vaccines are avoided during treatment, and killed vaccines are usually timed 7 to 10 days before a cycle. The timing grid is in the vaccine calendar.
Five things to set up before the first dose
- A digital thermometer and a paper log with columns for day number, temperature, food eaten in grams, and stool score 1 to 7.
- Antiemetic and probiotic supplies at home on day 1, not ordered on day 4 when the cat is already drooling.
- A baby scale, used twice weekly, with the same box every time.
- The clinic's after-hours number on the fridge, plus the neutrophil threshold that means go now.
- A written list of what the cat actually eats, so a 3 day appetite dip can be met with the right food instead of a random switch.
Most cats on feline protocols never need hospitalization. The ones that do are almost always the ones where a day 4 vomiting bout was watched for 2 days before anyone called.
Frequently asked
Will my cat lose its coat during chemotherapy?
Rarely. Cats differ from dogs and people in that most protocols cause no visible hair loss, though whiskers may thin or regrow curly after 2 to 3 cycles and the coat can look dull for 2 to 3 weeks. Large bald patches usually point to a skin problem, a reaction at the injection site, or over-grooming from itch, and deserve a photo sent to the vet rather than a wait-and-see approach.
How do I know whether vomiting is a side effect or an emergency?
Count and time it. One or 2 episodes on days 3 to 5 with the cat still eating is usually manageable with the prescribed antiemetic and a bland diet for 2 days. More than 3 vomits in 24 hours, any vomit with lethargy, a temperature of 39.4 C or higher, or refusal to eat for 24 hours means call the clinic that day, because dehydration in a cat on chemotherapy moves fast.
Is it true that treatment stops if the white count drops?
The dose is usually delayed, not cancelled. Vets commonly hold a cycle when neutrophils fall under 1,500/µL and recheck in 3 to 7 days; counts typically recover by day 14 to 21. A dose reduction of 20 to 25 percent or a switch to a gentler drug follows if the drop repeats across 2 cycles, and only a grade 4 drop under 500/µL usually ends that drug.