Penicillin itself is completely synthetic.
The reason that some might think it’s not vegan is because it’s tested on animals.
Even though most of the animal testing is long done, there are still researchers testing it in niche situations.
For example, here’s a study from 2019 where researchers induced epileptiform activity in rats.
Does Animal Testing Make Medicine Not Vegan?
This is not a unique problem, almost all medicine is legally required to be tested on animals.
When it comes to medicine like advil, which you can usually pass up at the trade-off of some discomfort, you can argue that it’s not necessary to take it.
But penicillin is different, it’s often life-saving.
The most commonly cited definition of veganism is along the lines of:
Do as little harm to living beings as far as reasonably possible.
Using penicillin is not going to have any effect on animals that it’s tested on, as sad as that is.
It’s not reasonably possible to not use penicillin in a time of need.
To summarize, just about all vegans would say that penicillin, like most other medicines, are vegan.
One day, hopefully we will live in a world where animal testing isn’t necessary, but sometimes you have no other choice but to accept it.