Top rack, and check the recycling number
Dishwasher-safe plastics exist, but they belong on the top rack, farthest from the heating element in the bottom. The heating element radiates enough heat to warp or melt plastic that drifts down to the bottom rack, so placement matters more with plastic than with anything else.
The fastest tell is the small recycling number stamped on the bottom. #5 (polypropylene, PP) and #2 (HDPE) are the most heat-tolerant and generally top-rack safe. #1 (PET, most disposable water bottles), #3 (PVC), #6 (polystyrene), and #7 can warp or leach and should be hand washed or not reused at all.
The dishwasher-safe plastics
Tritan copolyester — the clear, tough plastic used in Nalgene bottles and many modern tumblers — is dishwasher safe and BPA-free, though the manufacturer still recommends the top rack. Polypropylene (#5) food containers and lids, including most Tupperware, are top-rack safe; older or bargain containers warp faster, so watch for a #5 stamp.
Medical-grade silicone — technically a rubber, not a plastic — is fully dishwasher safe and even boil-safe, which is why Comotomo baby bottles can go straight in. Silicone lids, straws, and molds are all machine safe.
The plastics to hand wash (or skip)
Disposable #1 PET water bottles aren't built for reuse and definitely not for dishwasher heat — they warp and can leach. Melamine dinnerware (hard, colorful, lightweight 'plastic' plates) is a special case: the heat can cause it to release melamine and it dulls quickly, so most makers say hand wash. Anything with a foam or foil layer, a printed graphic you want to keep, or an electronic part (like a filter housing) stays out of the machine.
Water filter pitchers are a common mistake — a Brita reservoir can warp in the dishwasher, so hand wash the pitcher and never machine-wash the filter itself.
What happens when plastic goes wrong in the dishwasher
Warping is the big one: a lid that no longer seals, a container that won't stack, a bottle with a wavy rim that leaks. Clouding and a permanent hazy film are next — sometimes hard-water mineral buildup you can vinegar off, sometimes heat damage that's permanent. And a melted piece that touched the heating element can release fumes and ruin a cycle.
The safe habit: recycling #5 or #2, top rack only, air-dry or low-heat dry, and hand-wash anything precious or unlabeled. For bottles specifically, see our guide on putting water bottles in the dishwasher.