Benzalkonium chloride in pilocarpine?

Question

Why is benzalkonium chloride used as a preservative in pilocarpine hydrochloride, as opposed to other preservatives?

Answer 1

I would hazard a few good guesses:

1. inexpensive

2. good safety and efficacy profile

3. few adverse reactions

4. it's a detergent, which may help other ingredients stay in solution/suspension

5. pKa is closest to that of natural tears/saliva

6. broad spectrum of activity

Answer 2

Several reasons (most important shown below), although it isn't the only possible antibacterial which could be used.

It has fairly rapid action (important to kill quickly as eye drops are sterile products)

It has a good spectrum of activity

It is not very irritant (important for eye drops)

It is compatible with the product, both cation materials with chloride anion