First of all, OTC meds are NOT listed among the first aid and medical equipment and supplies that you must provide to your employees under OHS laws. But some employers choose to do so voluntarily.
The Q of whether that’s a good idea is a somewhat controversial one and I’ve seen strong arguments on both sides. For example, Alberta OHS guidelines say DON’T do it http://work.alberta.ca/documents/OHS-bulletin-fa014.pdf ; Mayo Clinic says YES, do do it https://www.mayoclinic.org/first-aid/first-aid-kits/basics/art-20056673
And getting to your Q, these are the liability risk you face if you do make OTC drugs available to your employees:
The employee may have an allergic reaction
There may be side effects, e.g., drowsiness
The meds may be addictive
The employee may actually be an addict
The employee may feel like he/she HAS to take the stuff
My advice:
If you do provide OTC meds, require that employees self-medicate–DON’T administer the drugs yourself
Stick to small quantities, not bulk supplies
Individual packets
Ask your OHS/WCB regional office or liability insurer what they think