Theme of the Week: When Does 1 Plus 1 Not Equal 2?

Medications often come in many different strengths - most of the time, when one strength is not available, a lower strength can be dispensed and instructions are adjusted accordingly to provide the same dose. However, here, we highlight some unique situations where substituting with another strength is not feasible.

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1) Amoxicillin-clavulanate (Clavulin): As you may know, this two-drug combination antibiotic tablet comes in 250/125 mg and 500/125 mg. Although you may be tempted to substitute 500/125mg with 2 tablets of 250/125 mg, this may actually increase adverse effects such as diarrhea because the clavulanate portion (125mg) is fixed in both tablets - so 2 tablets is actually not the same dose!

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2) Semaglutide tablet (Rybelsus): This anti-diabetic tablet comes in 7 mg and 14 mg tablets. Patients are usually titrated from 3 mg to 7 mg to 14 mg. However, it is important to note that 2 tablets of 7 mg are NOT equivalent to 14 mg. The reason is the formulation - each tablet strength is co-formulated with 300 mg of salcaprozate sodium, an enhancer which improves semaglutide absorption. By taking 2 tablets of the 7 mg, this increases the overall content of salcaprozate sodium and affects the absorption.

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3) Leuprolide (Lupron Depot): this injectable product comes in 7.5 mg and 22.5 mg strengths. It is important to note that these injectable products are long-acting formulations. The 7.5 mg formulation lasts one month, whereas the 22.5 mg formulation lasts 3 months. It would be incorrect to give 3 injections of the 7.5 mg because although the total dose is the same, the release rate is different, resulting in greater exposure over the month.

Now you know!