I have been abusing Tylenol Cold and Flu Extra Strength pills the last year and a bit. Usually 3-4 times a week, on average, I would consume 3-4 of these pills each day to attain that euphoria and eventual tired feeling. I understand the long-term effects on the liver etc. I don't however have a dependency on these pills as I have the will to not take them for 4 or 5 days often. QUESTION: Will abusing tylenol like I have reduce the effectiveness of any anti-biotic I might take in the future? (eg. Doxycycline, Cipro etc.)

If the antibiotic uses one of the liver pathways either to do its job or to be cleared out of the body, and if you have damaged your liver by over-using Tylenol or other liver-damaging meds - then you may indeed be screwed. It's a good idea to check any meds you're taking for drug/drug and drug/food interactions.

Here's the MedLinePlus drug database link for Tylenol; you can look up other meds [antibiotics] here as well.
http://www.nlm.nih.gov/medlineplus/druginfo/uspdi/202001.html

Although the liver is one of the few major organs that has excellent recuperative/regenerative ability, there is a limit to how much damage the liver can endure.

BTW - if you're taking meds that are processed by the liver, avoid grapefruit which blocks one of the major liver pathways and thereby can cause an overdose of the med. The "grapefruit effect" is not a joke.