system
Given the following post, respond as if you're commenting on Reddit:
human
Why do some medication come with instructions like 'take with a meal' and how does not following these instructions change the effectiveness of the drug.

Are the instruction 'take before a meal' or 'take after a meal' or 'take with lots of water' there for the same reason as 'take with a meal' or are they different instructions for different reasons?
gpt
It depends on the drug, and yes, it's there for a lovely reason.

We can use several examples to detail this.  

Ibuprofen should be taken with some type of food because it can irritate your stomach, and in long-term, severe cases can cause ulceration.  Other medications (penicillin based drugs, in general) have less absorption with food.  A drug like levothyroxine needs to be taken ~1-2 hours before meals for absorption purposes and because divalent cations (magnesium/calcium/etc) can bind to levothyroxine and essentially "inactivate" the drug.

On the flip-side, a whole host of medications should be taken with food to ensure delayed gastric emptying so that the drug has more time to be absorbed.

Or...it can depend on the pH of the stomach.  Some drugs tend to have a higher concentration, when absorbed, if the stomach has a more acidic environment or a more basic environment for solubility purposes.  Posaconazole, for instance, can be taken with Coca-Cola because it is more soluble in an acidic environment.  Griseofulvin is more soluble in a fatty environment (fatty foods).

Edit: And grapefruit.  Certain drugs (cholesterol lowering) have decreased metabolism when taken concomitantly with grapefruit.  This leads to an increased risk of adverse drug effects.

There's a whole host of reasons as to why there are specific reasons for these types of instructions.  I've only touched on a few of them.