Probably not, otherwise they'd have just asked for a fit note.
Fit notes are intended to cover short-medium term absences/changes to duties. Assuming you don't normally work from home, you're asking for a significant change to your usual work for a long period of time.
Your employer may also be concerned that if you're too drowsy to drive, that you shouldn't be working at all, and might want to suggest amended duties/hours etc.
OH costs hundreds of pounds and takes time - it could take weeks for your appointment to come through (usually a telephone call), they'll need to contact your GP anyway, and their report could take another couple of weeks to come through. Given that you've told them it should take around eight weeks to adjust to your meds, an OH referral feels like a waste of everyone's time/money in this instance.
A letter to your GP would likely be a long the lines of...
- Is Employee taking X medication?
- Would the side-effects make it dangerous for her to drive to work?
- Should she be able to complete her normal duties at home?
- How long do you expect the side effects to last?
They're not asking anything you haven't already told them, they're just getting confirmation from a professional. I'm not sure what you'd find invasive about that, unless of course you haven't been honest with your employer about something.
Usually your GP will contact you when they receive the request; you can tell them at this point that you only consent to that specific information being shared with your employer.