Biochemistry Questions / Describe the mechanisms of action of selective serotonin reuptake inhibitors (SSRIs) and the reasons for their slow onset of efficacy.

SSRIs have a slow onset, as they cause changes in the 5-HT receptor, causing the desensitisation of the receptor and the subsequent decrease in its activity. SSRIs also change the expression levels of the 5-HT receptor, downregulating the genes coding for it - this also takes time, slowing the onset.

Due to the changes in gene expression levels, it takes time for SSRIs to begin to have an effect.