Compounded Cat Medicine: 4 Forms, 3 Stability Traps

Cats refuse tablets more reliably than any other species, and nobody can split a 10 mg tablet into 8 equal parts for a 4 kg cat. Compounding exists for 2 reasons: the amount has to be small, and the medicine has to be palatable. Both reasons create stability problems, and those are controlled by the pharmacy rather than the vet.

Four forms cover nearly every case: oral suspension, transdermal gel, capsule or powder, and a flavored chew. Each fails in its own way, and 3 parameters decide whether the syringe you use on day 12 still delivers what the label promises.

Why a 4 kg cat breaks standard packaging

A tablet made for a 70 kg adult contains 10 to 500 mg. A cat needs a fraction of that, and the fraction is where errors start: a 1/8 split of a 10 mg tablet gives 1.25 mg on a good day and 0.6 or 1.9 mg on a bad one. Compounding moves the arithmetic to a pharmacy that dilutes once and fills 30 identical units.

Palatability is the second half. A cat that tastes bitterness will foam, drool and hide for 2 hours, and roughly 1 in 3 doses ends up on the floor or the fur. A flavored liquid at 0.1-0.5 mL per dose is easier to place than a fragment of tablet, and easier to confirm as swallowed.

The four forms, and when each one wins

Oral suspension wins for chronic daily medicine in a cat that will not take pills. Transdermal gel wins when the cat fights the syringe and 2 people are not available twice a day. Capsules and powders win for bitter actives and for 2 medicines combined in 1 unit. Flavored chews win for short courses and for cats that already take treats.

1. Form	2. Typical fill	3. Storage	4. Use within (days)	5. First sign
Oral suspension	10-30 mL bottle, 0.1-0.5 mL per dose	2-8 °C, fridge	14	Layer that will not shake out
Transdermal gel	10 mL syringe, 0.1-0.2 mL per dose	15-25 °C, away from light	30	Gritty texture or a greasy pinna
Capsule or powder	100-300 mg per capsule, 30 units	15-25 °C, dry	90	Caking, color shift
Flavored chew	1-2 g per piece, 30 pieces	15-25 °C, sealed	90, 30 once opened	Hardening, rancid smell

Oral suspension: the 14-day rule

Any water-containing liquid is a growth medium. The working limit most pharmacies print is 14 days at 2-8 °C, and that clock starts at mixing, not at opening. Shake for 10-15 seconds every time, draw with the oral syringe that came with the bottle, and never top up a half-empty bottle with a fresh one.

Two details decide potency. First, does the label give a concentration in mg/mL rather than a ratio, because 1:10 means nothing at the syringe. Second, is the suspension uniform after 5 minutes standing, since a settled drug means the last 3 doses are water.

Transdermal gel: 30 days and one ear

Gel goes on the inner pinna, alternating ears, and the site is not touched for 5 minutes. Absorption varies by drug and by skin, so a transdermal route is a poor choice when the therapeutic margin is narrow or when the cat's ear is crusted or haired. Expect 30 days from mixing and a loss of effect in the last 5.

Ask the pharmacy whether it measured penetration for that specific drug, or whether it is using a general-purpose base. The general base is the common answer, and it means the route is a convenience, not an equivalence.

Capsules and powders: 90 days, no water

Non-aqueous forms hold 90 days at 15-25 °C, which is why they suit long courses. Powder mixed into food must be eaten within 30 minutes, because a cat that grazes for 3 hours leaves the drug at the bottom of the bowl. Never mix into a full day of food, and never into a bowl shared with another cat.

For a cat on 2 medicines, ask for 1 capsule rather than 2, then confirm the pharmacy weighed both actives separately. Combined capsules are where a 10% error in one ingredient becomes a 2-week problem.

Flavored chews: 3 flavors, about 2 kcal

Chicken, fish and malt are the 3 bases that get accepted most often, and each chew runs about 1-2 kcal, which matters for a cat on a 200 kcal day. Order 5 pieces first, test acceptance over 3 days, then order the 30-piece batch.

Chews also hide a trap: the flavor can mask a drug that has degraded. Smell the batch when it arrives, and compare the color of piece 1 with piece 25.

The 3 stability traps: heat, light, days

Heat is the first. A suspension left on a windowsill at 32 °C loses potency faster than the label accounts for, and a car in summer reaches 50 °C in 20 minutes. Light is the second: clear bottles degrade light-sensitive actives within hours, so amber glass or an opaque sleeve is not decoration. Days are the third, and the one owners ignore, because a bottle that looks fine on day 20 is 6 days past its limit.

One more parameter deserves a line on the label: pH. A liquid buffered to 5-7 keeps most actives stable; a syrup at pH 3 accelerates hydrolysis and turns a 30-day plan into a 10-day plan.

How to vet a pharmacy in 10 minutes

Ask 3 questions at the counter. Does every batch get weighed on a balance readable to 1 mg? Is a sample of each batch kept for 90 days? Does the label carry the beyond-use date, the concentration and the storage temperature in writing? A pharmacy that answers all 3 without checking a folder is a pharmacy that has a system.

Then look at the bottle in your hand: name, strength, volume, date mixed, date to discard, and the words keep refrigerated or keep dry. Five of those 6 lines missing is a reason to call the vet before the first dose. Dosing routines also differ by breed and body type, and the York Chocolate and Ukrainian Levkoy calendars show how many daily medicines each one tends to carry; the cost tool prices a compounded course against a standard one.

Five steps before the first dose

Ask for the concentration in mg/mL and write it on the fridge note.
Mark the discard date on the cap on the day you collect it.
Test 5 units of any new flavor before ordering 30.
Keep the oral syringe with the bottle, never in the kitchen drawer.
Photograph the label so a refill request does not depend on memory.

Frequently asked

Is a compounded liquid as effective as the original tablet?

It can be, but only if the pharmacy states the concentration in mg/mL and the beyond-use date, and only if the base was validated for that drug. Ask whether the formula comes from a published reference or was improvised, and ask what the measured potency was on the last batch.

Why does my cat's transdermal gel stop working after a month?

Transdermal absorption depends on an intact inner pinna and on the base, and most pharmacies use a general-purpose base rather than one tested for that drug. Effect commonly fades in the last 5 days of a 30-day batch, so a return of symptoms at day 25 is a storage and formulation issue before it is a dosing issue.

Can I mix a capsule into a whole day of wet food?

No. Powder mixed into food should be eaten within 30 minutes, and a cat that grazes leaves most of the dose at the bottom of the bowl. Mix into a small spoonful given first, confirm it is eaten, then offer the rest of the meal, and never use a bowl shared with another cat.