chapter 3

• dosage - amount of medication given during a certain time interval
• despite efforts to optimize dosing, allergic reaction and other potentially side effects occur. the goal of optimum dosage is to reduce that while achieving clinical cure
• factors to consider in selecting best dose and dosage interval:

  1. in children, dose is based on the patient's mass
  2. 12 years above will typically have a single standard dose regardless of the mass
• additional consideration is how drugs are metabolized and eliminated from the body

  1. patient with liver/kidney dysfunction may experience reduced drug metabolism or clearance resulting in increased drug levels which may lead to toxicity and make them prone to side effects
• some drugs are dose dependent, they are more effective when administrated in large doses to provide high levels for a short time of infection. others are time dependent, they are more effective when lower optimum levels are maintained over a longer period of time
• more consideration is the half life-- rate at which 50% of a drug is eliminated from the plasma

  some drugs have short half-life (1 hour) and must be given multiple times a day. others have half life of exceeding 12 hours and can be given as single dose for 24 hours. although longer half life can be considered an advantage, it can also be a concern when it comes to side effects as they might remain toxic for longer time
• the route of administration is the method used to introduce a drug into the body is also an important consideration
• drugs can be administered orally, it is the preferred ones as they can conveniently take these at home. however some drugs cannot be easily absorved from GI tract into the bloodstream. these drugs are often useful for GI tract diseases such as tapeworms treated with niclosamide or for decontaminating the bowel as with colistin
• some drugs that are not absorbed easily like the bacitracin, polymyxin, and several antifungals are available as topical preparation for treatment of superficial skin infections.
• patients may not also be able to take oral medications because of their illness, when this occurs, drugs are administered intravenously. plasma levels achieved by intravenous administration is substantially higher than levels achieved by oral or intramuscular administration
• two antibacterial drugs may be administered together as they provide a synergistic interaction that is better than the efficacy of either drug alone. eg. trimethoprim and sulfamethoxazole. these two only provide bacteriostatic inhibition, but when combined, they are bactericidal
• antagonistic interactions produce harmful effects. it occur at two antimicrobials or between anti microbials and non microbials. the effects include loss of drug activity, decreased therapeutic levels due to increased metabolism and elimination or increased potential for toxicity due to decreased metabolism and elimination. some antibacterials are not absorbed most effectively from the acidic environment of the stomach. if a patient takes antacids, it decreases the effectiveness in treating infection. studies also shown association between antimicrobials and failure of oral contraceptives