As long as your pharm compounds are chromophores I think all you have to do is to use a thermostatized UV to determine the max absorption wavelenght of each compound and follow the decrease of each peak intensity over time. If absorbance is too high to consider valid the lambert-beer law and you do not know the molar absorptivity of the componds, you may have to dilute their solution or use very thin cells (1mm or so) to garantee that Abs aprox. proportional to concentration
I have worked with the spectropfotometric analysis of tetracyclines and Enrofloxacin:
20-BORGHI, A.A.; SILVA, M.F.; ARNI, S.A.; CONVERTI, A.; PALMA, M.S.A. - “Doxycycline degradation by the oxidative Fenton process”. Journal of Chemistry, v. 2015, Article ID 492030, 9 pages. Doi: 10.1155/2015/492030.
BORGHI, A.A.; OLIVEIRA-NASCIMENTO, L.; STEPHANO, M.A.; SOUZA, P.M.; CONVERTI, A.; PALMA, M.S.A. – “Cytotoxicity of Doxycycline effluent generated by the Fenton Process”. The Scientific World Journal, v. 2014, Article ID 439461, 5 pages, 2014. Doi:10.1155/2014/439461.
BORGHI, A.A.; PALMA, M.S.A. – “Tetracycline: production, waste treatment and environmental impact assessment”. Brazilian Journal of Pharmaceutical Sciences, v.50, n.1, p. 25-40, 2014.
FRADE, V.M.F.; DIAS, M.; TEIXEIRA, A.C.S.; PALMA, M.S.A. – “Environmental contamination by fluoroquinolones”. Brazilian Journal of Pharmaceutical Sciences, v.50, n.1, p.41-54, 2014.