There is many reasons why a farmer would dispose of a remedy. When this happens you can update your remedies cabinet in Herdwatch so it doesn't show there anymore for use on your herd.
Emptying A Remedy From the Remedies Cabinet
> Tap on Compliance.
> Tap on Remedies & Treatments.
> Tap on Remedies Cabinet.
> Click on the Remedy thats no longer in your cabinet.
> Tap Mark Remedy as EMPTY - Continue.
The remedy will now leave the remedies cabinet and will go into your empty remedies list.
Note: You can view all your empty remedies by tapping the Magic Button (top right) and tapping "Show Empty Remedies".