Base-Salt-Ester Conversion

Last Review: June 2005

There are several active pharmaceutical ingredients (drugs) that are offered in the market in different chemical presentations or forms. In other words, some drugs are available in their base form as well as in many different salt forms and/or ester forms. Nevertheless, the simple mathematical conversion of drug molecule weights will not be the correct procedure in all cases.

In the event there is a commercially available product in some form on the market, before performing calculations and compounding a prescription, the pharmacist must first refer to the commercially available product of that drug to see how it is expressed, i.e., as the base or salt or ester. As an example, albuterol sulfate is available in an inhalation solution labeled as 1.25 mg/3 mL as albuterol (equivalent to 1.50 mg albuterol sulfate); therefore, it contains 1.50 mg albuterol sulfate per 3 mL that is equivalent to 1.25 mg of the albuterol base per 3 mL). Consequently, it is very important to determine if the intent is to express the drug concentration as the base, salt or ester form. Examples of different types of conversions follow.

This database provides practical information on how to make conversions in converting base to salt or ester and vice-versa.

Below are some examples that illustrate some important points in this database:

  • When the commercially available product for that drug is expressed in terms of the base of the drug - See Example 1
  • When the commercially available product for that drug is expressed in terms of the salt or ester of the drug - See Example 2
  • When the commercially available product for that drug is expressed in terms of the salt or ester of the drug and different salts and conversion of different salts or esters of the drug is done to the referenced salt. - See Example 3
  • When different dosage forms contain different salts or esters of the drug. - See Example 4

Example 1:  When the commercially available product for that drug is expressed in terms of the base of the drug
 

When you look for Fluoxetine in this database and find the information that:

For each mg of fluoxetine, you may use 1.12mg of fluoxetine hydrochloride; it means that the commercially available product for this drug uses FLUOXETINE as its reference and NOT fluoxetine hydrochloride. Therefore, the pharmacist, before compounding preparations with fluoxetine, must calculate and convert the weight to the correct weight of fluoxetine hydrochloride.

Let's take as an example a prescription that calls for Fluoxetine 25mg capsules.

When consulting the BSE database, the pharmacist finds the equivalence:

for 1 mg of Fluoxetine
Ingredient mg's
Fluoxetine hydrochloride 1.12

To find the amount of fluoxetine hydrochloride per capsule needed to make 25mg capsules of fluoxetine:

25mg x 1.12 = 28mg of fluoxetine hydrochloride must be weighed.

 
Example 2:  When the commercially available product for that drug is expressed in terms of the salt or ester of the drug
  When you look for Phenylpropanolamine in this database and find the following information:

for 1 mg of Phenylpropanolamine
Ingredient mg's
Phenylpropanolamine hydrochloride 1.24*
* The commercial product is expressed as the salt form of the drug. No conversion is needed.

The value (1.24) in the table for the phenylpropanolamine hydrochloride is for mathematical purposes only once the legend below the table explains that NO CONVERSION IS NEEDED. This means that commercially available products with phenylpropanolamine are expressed in terms of phenylpropanolamine hydrochloride and NOT phenylpropanolamine.

Let's take as an example, a prescription that calls for Phenylpropanolamine 10mg capsules.

When consulting the BSE database, the pharmacist finds the legend explaining that NO CONVERSION IS NEEDED. Therefore, the pharmacist will weigh 10mg of phenylpropanolamine hydrochloride per capsule.

 
Example 3:  When the commercially available product for that drug is expressed in terms of the salt or ester of the drug and different salts and conversion of different salts or esters of the drug is done to the referenced salt
  When you look for propoxyphene in this database, you will find the following table:

for 1 mg of Propoxyphene
Ingredient mg's
Propoxyphene Hydrochloride 1.10*
Propoxyphene Napsylate 1.67**
* The commercial product is expressed as the salt form of the drug. No conversion is needed.
** The commercial product is expressed as the salt form of the drug. Conversion of different salts is done to that referenced salt in the commercial product.

The commercially available product in this case is expressed in terms of propoxyphene hydrochloride. Therefore, when using propoxyphene napsylate in a prescription, the pharmacist must first convert the amount to reflect propoxyphene hydrochloride.

Let's take as an example a prescription for Propoxyphene 50mg capsules. The pharmacy only has propoxyphene napsylate in stock

To find the amount of propoxyphene napsylate needed to make 50mg capsules of propoxyphene, the pharmacist must convert the amount to propoxyphene hydrochloride, as follows:

50mg x 1.67 = 83.5mg of propoxyphene napsylate must be weighed.

 
Example 4:  When different dosage forms contain different salts or esters of the drug
  When you look for Betamethasone in this database, you will find the following table:

for 1 mg of Betamethasone
Ingredient mg's
Betamethasone
Note: For oral dosage forms
1
Betamethasone dipropionate
Note: For topical dosage forms
1.28
Betamethasone Sodium Phosphate
Note: For oral, injectable and topical dosage forms
1.31
betamethasone valerate
Note: For topical dosage forms
1.21

Different salts and esters of some drugs, such as betamethasone, may be presented in different dosage forms. In this database, you will also find information on which drug to use in a specific dosage form and if there is a need for converting in that dosage form.