In proteomics, the changes occurring in cellular proteomes upon drug treatment are used to identify the drug targets and the mechanism of action. However, proteomes of cultured cells undergo also natural alteration associated with changes in the media, attaining a degree of confluence as well as due to cell division, metabolic and circadian cycles. These changes are implicitly assumed to be smaller in magnitude than the drug-induced changes that ultimately lead to cell demise. In this study, we tested this assumption experimentally by comparing the proteome dynamics of cancer cells during the first 48 h after treatment with methotrexate, paclitaxel or vehicle (equivalent to untreated control). Time series revealed considerable growth-related proteome changes in the control group, comparable in magnitude to drug-induced changes. The magnitude of these variations reveals an unexpected plasticity of the cellular proteome and reinforces the need, generally accepted in theory but not always followed in practice, to use a time-matched control when measuring drug-induced proteome changes.