These medications evolve their effects at the serotonin transporter. They increase the extracellular level of the neurotransmitter serotonin by inhibiting its reuptake into the presynaptic cell. They have no or only weak effects at other monoamine transporters, thus having little direct influence on the level of other neurotransmitters. That distinguishes them from the older tricyclic antidepressants (TCAs), thus they are named selective. SSRIs are considered to be considerably safer than TCAs, since the toxic dose is much higher and they are said to have fewer and weaker side effects and drug interactions.