Part of a rigourous quality control program is to verify colour, shape, size and weight of tablets/capsules etc. Either a) the company’s quality control is abysmal (because I refuse to believe that you caught an entire box of non-conforming lozenges while the QC lab got none!), or b) this is an intentional change in the appearance of the product.
In the case of a), you should be worried, not so much about tampering/poison, but simply over the fact that you have no guarantee that the product you’ve consumed is the product you bought. If you have no ill effects now, that’s good, and it’s very improbable that you will suffer any in the long term. It might be a lot of a new version of the same drug, mistakenly packaged into the old boxes, or it might be a lot of medicine of another type, with a similar appearance, again mispackaged. This shouldn’t be the case if the QC guys are paying any attention, but unfortunately, it’s been known to happen.
Imagine: the drug is made, then tested for impurities, correct dosage level, appearance, and any other criteria demanded by the FDA. It must conform (by law) to very rigid standards to pass Quality Control. In the meantime, the lot is stored in a warehouse. Once given the OK by QC, the stored bulk drugs are brought to packaging, they are placed in the blister packs/bottles/whatever package and outer boxes, and sent to the warehouse again, while a few boxes go back to the lab. Usually, the lab will verify “identity”, that is, they will check that if the box should be acetominophen, the pills in it DO have acetominophen in them. What many (otherwise good) labs fail to check, though, is the dosage at this point, the assumption being that the lot number has been tracked along with the pills, and if it passed earlier, after manufacturing, it will pass now. That is true (a correctly manufactured batch doesn’t suddenly become more or less potent), but it fails to take into the account that the 100mg and 1000mg doses might look the same/similar and they will both test positive in the identity test!. Packaging the lower dose in a high dose box is a mistake, but not necessarily a big deal. The opposite however… potentially fatal.
In your situation, I’d check to see if there is a picture of the intended lozenge on the box or on a website - they might still be using round blister packs to get rid of what they have left over in the warehouse, but the drug is now intended to be oval. If not, then I would stop taking the medicine and contact my pharmacist (even if this is OTC, they should know what it is and should look like) and the company until it is straightened out. Again, if it’s not an intentional change, a good QC lab will want to take your box and test the contents, to see if they can track down where/how the mix-up or problem occurred.
(btw, I am a former Quality Control chemist for a pharmaceutical company; I haven’t done any work for Cepacol, though!)