As you clarified in the comments, you are asking both about injections but also about "eating not in the normal manner" (e.g., pills) -- for a healthy person as well as a non-seriously ill patient. Here are some relevant sources, which are mostly found in cases of "eating not in the normal manner" for non-seriously ill patients (since things are more lenient for the seriously ill patient) and doesn't cover healthy individuals (since the rationale to inject/ingest forbidden things is not obvious).
The Tzitz Eliezer (quoted in Nishmat Avraham vol 2, p. 31) permits a non-seriously ill patient to take a medicine prescribed by a physician even if it originates from that which is forbidden. He also permits tablets or capsules made from [non-kosher] gelatine to be swallowed by a non-seriously ill patient and does not require them to be opened so that only the powder contained in them be swallowed (but he forbids lozenges that have to be sucked.)
In a longer exposition on the topic (pp. 65ff) he brings a number of additional opinions
- the Radbaz permits a non-seriously ill patient to eat all Rabbinicaly forbidden food [personal note: and even more so if not eating them in the usual manner or injecting them], however the Rashba, the Rivash, the Ran, the Shulchan Aruch and the Mishna Brura all disagree
- the Ketav Sofer writes a non-seriously ill patient may eat a forbidden food as a cure if he eats it in an unusual manner of if he mixes it with something bitter, so that he does not enjoy the taste. This is true whether the food is only forbidden to be eaten or even if it is forbidden to have any benefit from it. If he does not need the food for a cure but only to strengthen him, he should act strictly and not eat it, even in an unusual manner, since by eating it he gives it importance. On the other hand, since eating it in in an usual manner is only a Rabbinic transgression, one may act leniently if necessary
- the Pri Megadim writes that it is known that a non-seriously ill patient may be fed rabbinically forbidden food if he eats it in an unusual manner [e.g., pills or injections]. If, however, there are two different sorts of such foods available, the one which involves a lesser degree of prohibition is given to him
In cases where the substance to ingest or inject have first undergone a chemical change such that it is no longer edible (even if it is still edible to animals though not fit for human consumption), the Achiezer and Chavot Da'at permit it however the Sha'agat Aryeh forbid it for the non-seriously ill patient (vol. 2, p. 30).
Remember to always CYLOR if this becomes relevant in real life.
PS. For the sake of clarification, since it was asked in the comments, this answer focuses on non-seriously ill patients since the halacha is more lenient with seriously ill patients, but this question was explicitly focused on not using the leniency of pikuach nefesh (saving a life).