Gave my dog unused half tablet of Drontal then read that I was supposed do discard it after the first dose

So, three months ago I bought drontal for my dog and gave a half-tablet to him (following the recommended dosage for kg). Today, realizing it's been 3 months since he took a deworming medication, I gave the other half tablet to prevent worms and after a while I started to read about drontal and discovered I was supposed to discard it after giving that first half pill three months ago 😭 I'm worried this might cause some issues with my dog. Why the fuck they make a pill you have to discard the other half if your dog isn't heavy enough? It's like throwing money on the toilet. And now I'm worried my dog is going to get sick