system
Given the following post, respond as if you're commenting on Reddit:
human
If I were to drink something that is usually injected (morphine, heroin etc.) would the effects be the same as if I were to inject them? If not, why?
gpt
It depends. The concept you are referring to is "bioavailability". A drug has a bioavailability of 100% if exactly the same amount gets into your blood stream if you eat it, if you were to IV it.

However, very few drugs have a bioavailability of 100%, and the reason for this comes to to the interplay between anatomy and physiology.

When you eat anything (but in this case a drug), it goes from your mouth, into your stomach and then into your intestines. Absorption of the drug begins in earnest only once it enters you interestines. The reason for this is because 1) Your intestines have a lot of surface area for absorption 2) then have specialized transporter processes for absorbing things and 3) they have large blood supply, sucking away anything that is absorbed. This blood supply goes to one place, and one place only: your liver.

Basically everything that you eat, that ends up enters your blood supply gets sent to the liver. And the liver is prepared for this. It is a honeycomb of blood vessels, meaning that there is a huge surface area of the liver that is exposed to your blood. And the cells of the liver are packed with enzymes ready to chew up chemicals. There are literally 100s of enzymes at play here. A particular type of enzyme may target a few specific chemicals, or it may be able to chew up a vast array of different compounds. But the point is, that anything you eat has to pass this gauntlet that is the liver and its enzymes. The gauntlet is called "First Pass Metabolism".

Some chemicals can pass through the liver very well. Indeed, drug companies generally try to only work with drugs that do. But some drugs are absolutely savaged in the liver (low bioavailability). Most drugs fall in in the middle range. Morphine has a bioavailability of 20-40% meaning that 60% of it is destroyed by the liver. Ethanol about 50%. You can usually find out the bioavailability of any given drug just by googling "bioavailability <drug name>"