Pharmaceuticals and their metabolites are present in the environment, with levels expected to rise in the future. The effects on off-target organisms are still mostly unknown and of increasing concern, as are combination cocktail effects. Microalgae are of importance to ecosystems due to being primary producers and responsible for approximately half the global photosynthetic activity. Additionally, in some algal species a detoxification ability has been uncovered, linked to the enzymatic metabolism of cytochrome P450. However, the molecular mechanisms involved in the xenobiotic metabolism of algae are poorly studied, especially regarding cocktail effects. The aim of this study was to investigate the cocktail effect on growth inhibition and gene expression in Raphidocelis subcapitata, from environmentally relevant levels of pharmaceuticals. This was achieved by performing a growth inhibition test, according to the OECD 201 guidelines. Four of the most prescribed pharmaceuticals in Sweden, paracetamol, metoprolol, omeprazole and diclofenac were tested as a cocktail and compared to metoprolol exposure. Test concentrations ranged from naturally occurring levels up to that of a tenfold. In addition to optimization of RNA extraction, a transcriptomic analysis was performed on cocktail treated groups to evaluate expression of cytochrome P450. For all concentrations, metoprolol exposure resulted in algal growth inhibition, while cocktail exposure surprisingly promoted growth. Here the lowest and medium concentrations tested yielded a downregulation of the gene, while the highest concentration elicited an upregulation. The prospect of predicting cocktail toxicity is discussed, likewise the possibility of drug-drug interactions in microalgae due to cocktail exposure.