You can (and should!) google this - the US government, specifically the US military did a large study on a large number of drugs about just this topic. They store A LOT of drugs in case of conflict, a lot more than normally needed, so naturally they encounter this very problem on the scale of hundreds of millions of dollars of expired drugs.
They found that pretty much ALL drugs are safe to use long after expiration - 2 years is nothing in this context. There only were less than a handful of very specific drugs that were unsafe.
The only thing that will have happened is that there is a little less actual "DMSA" in your DMSA - but not much if you kept it refrigerated. I would not worry one bit about it in this context (they were refrigerated and it's DMSA, not a critical molecule).