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The most likely method of administration that leads to toxicity from anaesthetic solutions is intravenous injection. When local anaesthetics are administered directly into the bloodstream, they bypass the normal absorption processes that limit systemic exposure. This can allow for rapid increases in plasma concentration, which can lead to systemic toxicity, manifesting as central nervous system effects such as seizures, cardiovascular disturbances, or even respiratory arrest.

In contrast, the other methods of administration typically have additional safety measures or physiological barriers that reduce the risk of immediate systemic toxicity. For instance, injections in a supine position or into vascular areas can be controlled better through proper technique to avoid inadvertent injection into a blood vessel. Similarly, using anaesthetic solutions with vasoconstrictors generally helps to limit the systemic absorption of the anaesthetic by constricting blood vessels at the injection site, thereby reducing the risk of toxic effects.