system
Given the following post, respond as if you're commenting on Reddit:
human
If I orally took a morphine tablet, what happens from when the pill lands in my stomach to make the it start relieving my pain/generating a pleasant feeling? Why is it more effective (and often preferred) to use the intravenous method?

I have been reading about active and passive transport, and I understand somewhat, but I am unable to put this into the context of something practical/everyday such as taking medication. I have used opiates as an example as I understand they can be taken orally, but that they are often injected so that less of the drug is wasted. Is it something to do with the pH levels in the stomach? If you could explain what happens from the moment the pill lands in the stomach, that would greatly assist me in understanding the process from the get-go.

Thanks in advance!
gpt
You're thinking on the wrong level when you're thinking about active/passive transport. This is simple pharmacokinetics.

When you eat a pill, the pill usually dissolves in the stomach, and the material enters the duodenum/rest of small intestines. From here it is absorbed through the endothelium of the intestines and into the blood of the hepatic portal vein. Here is passes through the liver and onto the rest of the body and hopefully the target organ. In the case of opiates, the target organ being the brain and spinal cord.

There are two primary reasons why a drug is given via a non-oral route. The first is because some drugs have poor oral 'bioavailability'. This could mean the are rapidly metabolized in the walls of the intestines/liver (first pass metabolism). The opiate Buprenorphine is an example. It could also mean they are poorly absorbed from the GI tract.

The other primary reason why drugs are given non-orally is because of timing. If someone is in a lot of pain, expecting them to way 20 minutes for a drug to be fully absorbed from the GI tract might be too much to ask.