Phase Composition Analysis of Fixed-dose Combination Painkillers via X-ray Diffraction

Fixed-dose combination (FDC) describes medicines that include more than one active pharmaceutical ingredient (API) in a single dosage form. Combining different APIs can increase the drug’s potency or help to counteract side effects. A precise confirmation of the ratio of different APIs contained in FDCs is essential in both quality control and R&D; XRD is shown to be the ideal tool for such measurements.


According to the World Health Organization, headaches are one of the most common disorders of the nervous system with an estimated 50% of the adult population suffering from headaches at least once per year.1 Analgesics (painkillers) that are specially formulated to help with headache symptoms are commonly available as both over-the-counter and prescription medications. As headaches can be caused by different and sometimes multiple factors, the combination of multiple APIs that affect the body in different ways can improve pain relief. It has been shown that a combination of different APIs in a single dose results in more effective pain relief than the same dose of a single drug, and the addition of other co-analgesics, that offer no pain relief by themselves, can even increase this effect. Multiple studies show that caffeine in particular can increase the effectiveness of most pain killers to such a degree that the pain relief occurs in half the time or with half the dose that would be required without caffeine. In addition to faster and more potent pain relief, caffeine has far fewer negative side effects or associated health risks than most APIs that it typically replaces. 2,3 

In such fixed-dose combinations (FDCs), the ratio of the different analgesics in the product is of fundamental importance. Thus, the determination or confirmation of the phase composition is an essential step during development and in quality control and is typically performed using X-ray diffraction (XRD). In this application report, the phase composition of a commercially available anti-headache medication, that is an FDC of the three different components (acetylsalicylic acid, acetaminophen and caffeine) is determined. The presence of different crystalline and amorphous excipients is investigated and the ratio of the three components is finally determined through a quantitative fit with the Rietveld method.


1. World Health Organization, Headache disorders,, Published April 8, 2016. Accessed November 21, 2021.
2. Diener, H.C., Pfaffenrath, V., Pageler, L., Peil, H., Aicher, B. Cephalalgia. 2005, 25(10), 776-87.
3. Petersen, K.U. MMW - Fortschritte der Medizin. 2013, 155, 109–114. 

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