Fill a cup with vinegar and submerge your retainer. In a few minutes, brush the retainer thoroughly. If there’s a lot of calcium buildup, you may have to keep it submerged for closer to 20 mins. Denture cleaner or baking soda is also effective.
Be careful, though, as some retainers are made of materials that can damage the retainer. Check with your dentist or orthodontist if you are unsure.
Hot water rinsing may warp the retainer. Avoid it. Be careful with toxic chemical cleaners. Your retainer’s material is porous, soaking up the toxins. Remember you’re putting your retainer back in your mouth, so your cleaning supplies should be safe to ingest.