Publications

Published: 5 March 2015

Medicine Labels - Rubbing Salt into the Wound

Prescriber Update 36(1): 10
March 201
5

Key Messages

  • The salt form of an active ingredient can significantly influence the pharmaceutical properties of a medicine.
  • Some medicines have multiple salt forms of the same active ingredient.
  • The strength of the active ingredient stated on a medicine label is typically based on the active moiety (strength = active) but can also be expressed as the salt form of the active ingredient (strength = active + salt).


Prescribers are reminded that different salt forms have different doses.

Medsafe ensures that the strength of the active ingredient is clearly stated on labels as this is crucial to the safe and effective use of medicines. There is potential for confusion when the same active ingredient is present as different salts in different products with similar indications.

The selection of an appropriate salt form for an active ingredient is a critical step in pharmaceutical development. The salt form of the active ingredient is used to alter the physical or chemical properties of the medicine, such as solubility, pH, stability and shelf life, and bioavailability. This in turn can influence the dose form of the medicine.

An example of an active ingredient with different salt forms is metoprolol, a selective β1 receptor blocker indicated for the treatment of hypertension, angina and hyperthyroidism. There are two salt forms of metoprolol currently available in New Zealand; metoprolol tartrate (eg, Lopresor tablets) and metoprolol succinate (eg, Betaloc CR tablets). Metoprolol tartrate is formulated as an immediate-release tablet and the succinate salt form is an extended-release tablet. Both dose forms have slightly different indications and different dosing requirements. A fumarate salt form of metoprolol is also approved but not currently available in this country.

The strength of the active ingredient specified on a medicine label is typically based on the active moiety but can also be expressed as the salt form of the active ingredient. This is particularly relevant if a patient's prescription is changed to different dose form of the same active ingredient because different salts can require different dosages. For example, 95 mg of metoprolol succinate is equivalent to 100 mg metoprolol tartrate.

If in doubt, prescribers should refer to the product data sheet on the Medsafe website for full dosing instructions.