Lateral line cleaner is a safe chemical (for the drains, definitely not you). Buy it at home Depot and fill the drain. If the drain is backed up, that's fine, fill it till the drain is full. Follow directions on bottle. It will dissolve anything in the drain that is from your body or food, like hair, fingernail clippings, meat, veggies, grease. It won't dissolve things like metal or bone.
It was a fatberg in the sink vent. Not sure how it got there, that drain has always been slow since we moved in. I think I have it figured out now.
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