Why aren't some drugs available intravenously?

My niece is in the hospital and having some bad headaches after the treatment (can’t remember the name, but not important for this question) for ITP (low platelets). It seems that they cannot give Tylenol intravenously. Since she cannot keep anything down, this means that they’ll have to keep giving her morphene, which knocks her out completely.
This got my SIL and I wondering about the differences between drugs formulated for IV and the same drug formulated for taking orally. The nurse confirmed that there are different formulations (i.e., you can’t just take Tylenol for taking orally and put it into the IV drip), but she didn’t know why Tylenol wasn’t available for IV use.

1.) What are the differences between the IV and the oral formulations of drugs.

2.) Why are some drugs not available for IV use? Are there some drugs that must be absorbed through the stomach to be effective?


I don’t have an answer as to why Tylenol (acetaminophen) can’t be given intravenously, but there does seem to be way to take it if it’s not possible by mouth.

Acetaminophen suppositories.

There are all kinds of reasons that most drugs aren’t available in IV form. Some have to do with absorbtion - some drugs need to be broken down and absorbed in the gut to be effective. Some drugs, for chemical reasons, can’t be given into a vein - the Ph is too high or too low. There are some IV medications (IV dilantin, chemotherapy, TPN, etc) that can only go into the BIG veins in the neck or chest, not the smaller veins in the hand or arm, because they’re so irritating they need to be diluted rapidly in a high-flow situation.

There is much more science to giving IV medications than one might expect. Some drugs can’t be mixed with certain IV fluids (dilantin will crystallize if mixed with the wrong base fluid), some drugs aren’t compatible with each other, some can’t be given too quickly, some medications can’t go in plastic bags, only glass bottles, some can’t be exposed to light, etc. Some IV solutions will pull fluid out of the tissues; some will put fluid in. There are entire text books written about IV therapy and which fluid to use when. It’s a fascinating, but complicated, subject.

There are all kinds of ways to give Tylenol, including by suppository. If Tylenol isn’t effective, there are other injectable pain medications that are non-narcotic and effective - Toradol, for example. There’s a wide range of stuff available for pain relief between the Tylenol and morphine ends of the scale.

Yeah, IIRC, they had to give her a suppository at one point because of her inability to hold stuff down. (I’m just the uncle repeating third-hand info here.) I appreciate the answer, but this just introduces another question:

3.) What’s the difference in formulations between IV and Oral formulations, vs. the suppository formula?

IV medications need to be in a sterile, liquid form. The stomach and the rectum can tolerate a formulation that’s ‘clean’ as opposed to ‘sterile’. Introducing something directly into the bloodstream is a different matter, and sepsis is a concern. Oral medications also include a lot of extras such as binders, coatings and coloring agents. These help hold the pills together, make them easier to swallow and better tasting, and help them dissolve in the right place in the gut and at an even rate. IV drugs may have additives to adjust the Ph balance and make them less irritating to the veins.

Giving IV medication has to be done carefully, because once it’s in you can’t get it out. Give someone the wrong oral med and there’s time to act with ipecac, charcoal, antidotes, lavage and the like. Give someone the wrong IV med and the action is immediate - it’s in the entire system within 30 seconds or so. There is very little margin of error with IV meds - IV medications errors can kill, and very quickly. Not that action can’t be taken (depending on the circumstances) but the risk is so much higher.

Acetaminophen (Tylenol) gets transformed in the liver to a less toxic metabolite which can be excreted easily by the kidneys. That’s why if you have liver problems that will interfere with the transformation, they tell you not to take Tylenol - it doesn’t get excreted properly and can build up to toxic levels.

I suspect that’s the main reason that it wouldn’t be given by IV. Because it’s not passing right through the liver right away as it would if it was taken as a pill, it’s harder to keep the medication in the therapeutic range and below the toxicity threshold.

(But hopefully one of the DoperDocs will come by, so we can stop with the guesses and get some qualified answers!)

Some drugs actually need to pass through the liver to undergo biotransformation to activate them. The pill you take is sort of the pre-drug, and your liver enzymes do the rest of the work.