Our pharmacy is located near several pediatricians’ offices, and each of these pediatricians has a preferred mouth rinse for sore throat. One doctor writes a combination of lidocaine, antacid, and Benadryl for a sore throat. Depending on the cause of the sore throat, she may add nystatin, an antifungal medication, or acyclovir, an antiviral medication. A typical prescription will read: antacid 1 part, lidocaine 2 parts, Benadryl 1 part, and nystatin 4 parts; dispense 8 ounces. We have found one of the simplest ways to calculate the amount of each drug to mix is by using fractions. First we add up the total number of "parts," and this becomes our denominator or bottom number. For this compound, the denominator is 1 + 2 + 1 + 4 = 8. The numerator or top number is how many parts of that ingredient are used. For example, two parts of the mixture are lidocaine, so lidocaine is 2/8 of the mixture. To calculate how many ounces of lidocaine are needed, the fraction is multiplied by the total amount of mixture: 2/8 × 8 ounces 5 = ounces." — Kelly Smith, Pharm. D. Pharmacists often use fractions to determine how much of a certain medicine to mix for a given dose. In the example above, the pharmacist explained how she uses fractions to mix a particular medication that a local physician prescribes. Using this same example, how much lidocaine would you use if the physician wanted you to mix 16 ounces?