The singer signed a publishing deal with Warner/Chappell Music in 2011, which led her to write songs for artists like Girls Aloud and Icona Pop.

According to Lo, the tracks talk about her most intense love affair, from the happy beginning of the relationship to the part when she is trying to move on after the break up.

On 25 February 2014, "Stay High", a remix version of "Habits (Stay High)" by record production duo Hippie Sabotage, was released as the third single from the EP to commercial success in Europe and Oceania, peaking at number 13 in Sweden and reaching the top ten in Australia, New Zealand and the United Kingdom.

In 2006, after finishing high school, Tove Lo formed a math rock band called Tremblebee,[1] with whom she played "shitty bars" across Sweden until 2009.

[9] The following month, the artist independently released "Love Ballad" as her debut single, which gained attention from music blogs.

[19][21][22] On 23 September 2014, the EP was re-released, featuring a remix version of "Habits (Stay High)" by Swedish DJ Oliver Nelson and three out of the six original tracks.

[29][30] Alex Panisch of Out opined that the first five tracks "document the five stages of a relationship turning sour: good times, doubt, breaking up, coping, and still being hung up".

[30] Critics also noted that, unlike other artists' break up records, the singer admits in Truth Serum that she was the person who made a mistake that ended the relationship.

[31] Panisch commented that; "When female artists sing about breakups [...] they almost overwhelmingly write themselves in the position of the wronged party and their songs are about recovering and strengthening from the experience.

[30] Shanon Carlin of Radio.com stated that the singer "doesn't try and capitalize off her heartbreak by writing a female empowerment anthem that shames the guy who did her wrong.

[29] In an interview, Lo summarised the storyline of the EP; "The song 'Love Ballad' is about the moment you decide to give everything to another person, while 'Habits' shows what happens when everything is screwed up and you just want to freak out.

[8][41] "Paradise" is a jungle and drum and bass song with electronic elements;[18][42][43] it talks about the moment when Lo realizes that she needs and relies on her partner.

[52] Concerning the track's re-release, the artist stated that the label representatives felt that it "still [had] a lot left to give" and wanted to promote it properly.

[58] It similarly experienced success in Europe and Oceania, reaching number 13 in Sweden and peaking within the top ten in Australia, Netherlands, New Zealand and United Kingdom, among other regions.

[69] On 22 April 2014, Lo performed the songs from Truth Serum alongside her collaboration with Lucas Nord, "Run On Love", at live music venue Echoplex in Los Angeles.

[76] On 18 June, Lo made her first appearance on American television when she sang "Habits (Stay High)" on Late Night with Seth Meyers.

[77] A month later, she performed "Habits (Stay High)", "Out Of Mind", "Not On Drugs" and "Over" on American music television show JBTV.

[26] Furthermore, he ranked the record at number two on his list of the top ten singles of 2014, writing that, "Every track hits you like a hurricane – the pop hooks deployed like rock riffs as Tove excavates her darkest secrets".

[83] Ben Oliver of We Listen Hear called it an "excellent debut EP" and "a collection that not only showcases a sharp new talent but sets a new standard for emotional pop music".

[30] John Calvert of NME gave the extended play a mixed review; though he praised Lo's sincerity in the lyrics of "Habits (Stay High)", he criticized the production of Truth Serum, writing that it is not "anywhere near as boundary pushing, sonically – instead, it's dominated by anodyne, cliché-packed Scandi-pop".

[82] Tord Litleskare of the Norwegian publication Gaffa also gave the EP a mixed response, stating that the lyrics of the tracks were "silly and sad" and "too focused on heartbreaks and drugs".

[85] Camila Astorga Díaz of the Swedish edition of Gaffa wrote that Truth Serum "fulfils its role as background music while traveling", but considered that it lacks "that one thing that makes you want to play repeat".

[22] Peter Carlsson of Allehanda called it a strong debut but stated that it did not help Lo to stand out from other pop artists.

[86] Svenska Dagbladet's Sara-Märta Höglund compared the EP to Katy Perry's Prism (2013) and stated that, "though Lo knows how to make a hit, I have wished she had ventured out on a more stripped-down production and dropped the tiresome drug romanticism".

[90] In Australia, Truth Serum debuted and peaked at number 99 on the Australian Albums Chart for the issue dated 7 July 2014.