Maxidex Eye Drops use in dogs - safe or just ineffective if stored below recommended temperatures?

I am not a medical professional and this answer is not medical advice nor a substitute for medical advice.

At first, it could be helpful to estimate whether the eye drops could even have frozen in the temperature inside your fridge. Pure water freezes at 0 °C, but any dissolved impurities will generally lower the freezing point (for example, saturating water with salt lowers the freezing point from 0 °C to around -19 °C). What I would do - I would get some transparent, small container - like a vial or a disposable jug from pharmacy intended to hold urine samples - then I would drop a few drops of this medicine to it, close the vial and put it in the fridge to see whether the medicine freezes in this temperature. Afterwards, I would dispose of the vial/jug together with the drops in that bin at pharmacy where it is intended to throw away expired medicines.

Generally overheating medications is much worse that overcooling them. Lowering the temperature tends to slow down the rate of chemical reactions (including decomposition), while increasing temperature accelerates that rate. However, this simple relation doesn't take into account that the process of phase change in freezing/thawing could potentially change physical properties of the medicine.

According to this chemical data sheet of dexamethasone and other sources, it is practically insoluble in water (water solubility of 89 mg/L according to one of the figures in the data sheet). It makes sense because dexamethasone is a steroid and steroids are generally hydrophobic (insoluble in water, soluble in non-polar solvents like fats).

According to this source, the ingredients of Maxidex are:

The active ingredient in Maxidex Eye Drops is dexamethasone 1 mg in 1 mL.

Maxidex Eye Drops also contain:

• benzalkonium chloride as a preservative;
• dibasic anhydrous sodium phosphate;
• polysorbate 80 (Tween 80);
• disodium edetate;
• sodium chloride;
• hypromellose;
• citric acid monohydrate and/or sodium hydroxide use to adjust pH;
• purified water.

Polysorbate 80 and hypromellose are both emulsifiers, also known as emulgators, which means they are added to stabilize the mixture of immiscible substances - this, together with the instruction of shaking before use, strongly suggests that these eye drops are indeed an emulsion, probably dexamethasone and water in this case.

The problem is that many emulsions are not freeze-thaw stable. With some exceptions, emulsions are thermodynamically unstable and have a natural tendency to separate over time. For example, it requires a lot of energy input - in the form of vigorous stirring and shaking the container - to mix oil with water, but without any stabilizer such mixture quickly separates into two completely distinct layers. Stabilizer's function is to greatly slow down this process of separation. However, subjecting an emulsion to freezing-thawing cycle causes stress at the interface of immiscible phases and thus could facilitate aggregation of the dispersed particles into larger units.

If the suspended phase aggregates and separates from the solution as larger clumps, then it is a valid concern; it would change the physical properties of the medicine into something different that the producers intended. For example, the fact of separation would make it so that the concentration of the active substance will no longer be uniform in the liquid. It could be a problem because the safety and effectiveness of the treatment depends on using the specified dosage - but if the solution in not uniform in terms of concentration anymore, you risk dosing too little or too much; theoretically the first couple of drops could contain no dexamethasone whatsoever, then the next couple of drops could contain a few times more that you'd expect.

In the topic of shaking the eye drops before use - it is done to further make sure that the mixture is as uniform as possible. As said before, emulsions "want" to separate and do it slowly over time, so it is especially important if the drops have been sitting immobile for a long time (a few months?) on the shelf before use. However, I think we could certainly assume that during the recent time you were buying the drops and transporting them home, they have been moved around and at least somewhat shaken. It is definitely better to remember doing it manually next time, I don't think it was anything bad or catastrophic that you forgot the first two times - it just was not optimal.

Nonetheless, if you are still concerned about not shaking before use, here is my next idea about testing for yourself - you could leave the bottle of these drops to sit on a shelf undisturbed for, let's say, a week; then without shaking apply a few drops on some surface (I suggest some flat, transparent piece of glass) in one place, then vigorously shake the bottle for at least a minute and apply a few drops again in a different place on the surface - and then observe whether there is any noticeable difference in the appearance of these two samples (one more cloudy than another, one having different color, etc.), preferably using some bright light source to see more details.