Drain cleaner is not the most effective way to clean a toilet or to clear a blockage in a toilet. It is designed for dissolving blockages in the pipe structure below a sink, whereas a toilet usually has a U-bend. Because of this difference in structure, the drain cleaner is unlikely to reach the blockage.
Using drain cleaner on a toilet blockage may not be effective, which could lead to further chemicals being used. This can create potentially dangerous chemical reactions. Plunging the toilet may also be attempted, which could lead to drain cleaner splashing out of the toilet and potentially harming someone.
It is recommended to use a specialist toilet cleaning or unblocking product instead, or to call out a professional plumber if there is a blockage.