Even if the medication is what caused the spawn, which it most likely is, I don't think it is any reason for us to be too worried about the medication being very "dangerous". IME, it doesn't take much stress to trigger a spawning event. My clams have spawned several times, all stress induced. The last time was just from scraping coralline off the acrylic in my clam tank.
Barry, why do you recommend the water change and carbon after a spawn? I am guessing the sperm has the potential to rapidly lower O2 levels, especially when several clams go off at the same time. I suppose it could also cause a spike in the nitrogen cycle. FWIW, I've never done anything special after my clams have spawned. I always figured it would be good food for the corals. I suppose it is always better to be safe than sorry though.