2 layers when you ingest a substance to get through to main blood: intestine with its enzymes & barrier (or more rarely the stomach), then the liver with its enzymes. (gastric acid can affect some things too e.g bromelain where rodents have less acidic fasted ph vs humans)

I.p bypasses the need to go through intestine, just needs to clear the liver

so when injected i.p for example if a substance has a large molecule size and doesnt use an intestinal transporter, usually that would be well rejected before getting to the liver. but with this route it goes through.

some enzymes in common with similar activity level between intestine and liver. liver is generally more active but intestinal metabolism can be substantial.
intestine also has p-glycoprotein transporter which rejects some things back into the gut space

some substances might be pretty much the same where they dont get metabolised by enzymes much and have high transport utilizing transporters around (or small enough to get through tight junction barrier very well).
sometimes there are big differences
example, cmax was 4x higher with Deramciclane as i.p vs orally and 6x higher AUC https://pmc.ncbi.nlm.nih.gov/articles/PMC7412579/

The absolute bioavailability of deramciclane in rats was 3.42% after oral and 18.49% after intraperitoneal administration

Lenalidomide 4x higher i.p vs orally
then Tariquidar its close between i.p and oral
for something like doxycycline it wouldnt matter because oral bioavailability is so high

basically its better to find oral dosing (theres other problems too like conjugation differences but) when looking to try get an idea of dosing between animals and humans