sick, should i take the antibiotics?

YES. Antibiotics should always (unless otherwise directed by the prescribing Physician, especially if it's your gf's dad. He won't like it if you play fast and loose with your health and maybe his daughter's? Getting my drift? Also, doctors have large egos and don't like it if you don't follow instructions. Be glad you might have a doc in the family, a lot of people don't even have insurance. Okay, enough preaching :) ) The reason you should always finish antibiotics is that (even though his diagnosis is presumptive, it's more likely you have a viral infection and the antibiotics are more prophylactic in use <just in case?> indicated by a 5 day course as opposed to 7 -10 days), if you "do" in fact have a bacterial infection, and you fail to take the entire course of treatment...the bacteria will become resistant to the antibiotic you are presently taking. Then if the infection re - occurs you will have to take a stronger, more wide spectrum antibiotic which will not only kill the targeted bacteria (hopefully), but will also kill essential bacteria in your system possibly causing further difficulty (especially in the Gastro - intestinal tract). This is the reason you should always take the full course of antibiotics prescribed by a physician. Plus he's your gf's dad. MD's pay outrageous malpractice premiums, he's also risking being sued (though I'm sure you wouldn't do such a thing), if anything bad happened because he made a presumptive diagnosis over the phone for someone he probably doesn't even have a chart on. So not only did he do you a good turn, he also took a risk. It may seem like a little thing, but big things grow from little things. Do the right thing for everyone and finish your meds.

Good luck

He presumes based on your symptoms that you have a sinus infection. What you have to lose is the cost of the antibiotics ,if you have to pay out of your pocket and the possibility of a side effect or allergic reaction to the medicine. All considered if you are not one to have allergic reactions to antibiotics they will likely get you better quicker. A decongestant/antihistamine used in conjunction would be a good idea as most people with this have underlying allergies to pollens etc. Don't neglect maintaining good hydration and the basics. Get well.

if a doctor prescribed medication then you need to take the medication as it was prescribed for you