The sulfuric acid in drain cleaner can cause minor burns to the leaves and stem of the plant if only a few droplets of the drain cleaner spilled onto the plant. Some drain cleaners have stronger and more effective sulfuric acid concentrations than others, and stronger drain cleaners may burn plant parts more severely. Plants will more than likely survive burns, as long as they are minor and do not cover the majority of the leaves. Drain cleaner should be gently wiped from the plant's surfaces after spilling so it won't sit on the leaves and burn the plant further.
If the drain cleaner gets into the soil and burns the roots, or burns enough of the leaf surface that the plant cannot receive light and water properly, the entire plant can wilt. If the plant is severely effected and begins to wilt, you may not be able to save it.
Because drain cleaner is acidic, it can change the pH level of the soil. Some plants, such as blueberries and roses, require acidic soil to thrive, but others cannot tolerate soil with high acidity. If the soil acidity changes the plant may not thrive, and the plant may need to be transplanted to different soil if drain cleaner leaches into the soil.
A significant amount of drain cleaner can kill a plant. A significant amount would be enough drain cleaner to coat some or all of the leaves, which would lead to the death of the leaves and eventually the death of the plant. If the spill is significant, quickly rinse the plant off to remove as much of the drain cleaner as possible.