Mana is just like every other substance in the body
It's often said that poison and medicine are two sides of the same coin. Have too much or too little of anything in your body and it's a poison. What matters is the dosage, the form of the substance, and the means of getting and storing the substance.
Water is essential to life but a quick google of Dihydrogen Monoxide will show you a plethora of ways it can kill you.
You can easily and safely eat chicken soup but injecting it into your blood stream won't end well.
Oxygen is essential to life but, as others have pointed out, pure oxygen is something you shouldn't breathe.
Mana overload is a thing
There as been many stories where "Mana Overload" is a thing, as in having too much mana can make the user weak, sick, collapse, or even explode.
Each human may have a maximum amount of mana they can process/use at a time, which is their usual rate of use. Using an external mana crystal may force the body to exceed that natural maximum, causing them to get Mana Poisoning.
Mana from fetuses is unstable
As one grows, their body gains the ability to absorb and purify/stablelize mana naturally. However, a fetus relies on its mother to do the task. The mana extracted from fetuses may be unstable or impure (on something else), thus making it harmful to the body.
Process the mana after extraction and before selling/using it.
Mana outside the body can be contaminated
Having raw mana outside the body can make it "go bad" like food. Some miasma can contaminate the mana crystals. Just having mana exposed to air can make it react, similar to how metals oxidize.
To fix this, you can store mana in some specialized medium like pill capsules,
pokemon mana balls, etc. You can also purify the mana during production, removing contaminants that may have got in during extraction.
The medium to store the extracted mana is bad
Even if you have the best medicine, if the container for it damages the medicine, it can ruin the medicine. Improve your containers of mana.
If the body has a natural container to store mana, say a mana heart (different from the blood-pumping heart), you can extract mana hearts from humans (and may be other mana-using creatures) and use those as containers.
Mana inside the body is different from mana from outside sources
Mana inside the body can be naturally refined for use. However, the body can't process a large amount of mana from outside, meaning that mana can't be made safe for consumption by the body.
Find someway to refine the mana to make in safe. May be capture some humans and turn them into mindless mana refiners.