From psychology to economics there has been substantial interest in how costs (e.g., delay, risk) are represented asymmetrically during decision-making when attempting to gain reward or to avoid punishment. For example, in decision-making under risk, individuals show a tendency to prefer to avoid punishment than to acquire the equivalent reward (loss aversion). Although the cost of physical effort has recently received significant attention, it remains unclear whether loss aversion exists during effort-based decision-making. On the one hand, loss aversion may be hardwired due to asymmetric evolutionary pressure on losses and gains and therefore exists across decision-making contexts. On the other hand, distinct brain regions are involved with different decision costs, making it questionable whether similar asymmetries exist. Here, we demonstrate that young healthy human participants (Females:Males=16:6) exhibit loss aversion during effort-based decision-making by exerting more physical effort in order to avoid punishment than to gain a same-size reward. Next, we show that medicated Parkinson’s disease (PD) patients (Females:Males=9:9) show a reduction in loss aversion compared to age-matched controls (Females:Males=11:9). Behavioural and computational analysis revealed that people with PD exerted similar physical effort in return for a reward, but were less willing to produce effort in order to avoid punishment. Therefore, loss aversion is present during effort-based decision-making and can be modulated by altered dopaminergic state. This finding could have important implications for our understanding of clinical disorders that show a reduced willingness to exert effort in the pursuit of reward.