Anyone can drink an elixir or feed one to someone else. The Herbal Lore skill allows you to apply contact gels (to surfaces or weapons as appropriate). Three levels of the Alchemy skill allows you to use Alchemy Globes.
Many substances can be reversed by a specific antidote if taken after exposure to the substance.
The spell defensive Poison Shield will not stop ingested substances from taking effect, but will stop all other attacks with with the Poison qualifier such as Alchemy Globes. The racial skill Resist Poison may be used on any effect with the Poison qualifier no matter how it is are delivered, and is one of the only ways to stop an ingested toxin.
Note that contact gels which do not affect the metabolism (e.g. do not use the Poison qualifier) are not stopped by a Poison Shield and can only be removed by an appropriate cure—in some cases an Alchemical Solvent, in others a Purify or a Dispel.
You cannot have more than one active contact gel on your weapon or any other item at any one time, and only the most recently applied will be effective.