Her sister-in-law Julie, an ER nurse, told People that McAllister had taken a "combination of two different prescription medications, and Tylenol and Aspirin" on Saturday, when she was rushed to the hospital and put on life support.
"In the short time that we knew Lex, she was a lovely person inside and out," The Bachelor producers and crew said in a statement to E! News on Wednesday. "She will be missed by the entire Bachelor family, and we extend our heartfelt condolences to her family and friends."
McAllister, who was nicknamed "Lex," competed on The Bachelor: On The Wings of Love, which aired in early 2010 and starred Jake Pavelka. She was billed as a 25-year-old entrepreneur from Galloway, OH, at the time and was eliminated during the very first Rose Ceremony.
Although her stint on the show was short, Julie insisted McAllister was "very glad" she gave it a try. McAllister's goal for appearing on the ABC franchise was to find love but also to "empower women."
"I can vividly remember Alexa getting out of the limo on that first night," Pavelka toldPeople in a statement. "She was a pretty girl. I'm really sad. I feel horrible for her family and I can't imagine what they're going through. My prayers are with her friends and family."
Pavelka added on Twitter that McAllister was "beautiful" and his "heart breaks" for her mourning family.
Pavelka's ex-fiancee, Vienna Girardi, also expressed her "sincere condolences."
"My heart goes out to Lex's family during this difficult time," Girardi, the winner of The Bachelor's fourteenth season, told E! News. "I remember her being such a positive young lady who could light up any room."
Tenley Molzahn, the runner-up of Pavelka's season, tweeted kind words on Tuesday.
"Remembering Lex," the 31-year-old bubbly blonde wrote. "She was vibrant! I didn't know her well, but she was beautiful. Prayers to her family."
Molzahn, who also appeared on Bachelor Pad and Bachelor in Paradise, additionally shared an image featuring an inspirational quote on Facebook that reads: "Scream 'I am worthy' until the stars collapse upon your brilliance."
Molzahn captioned the image: "Hey you! You who are hurting... You who feel crushed by life... You who doesn't feel worth it today. I have something to tell you, you are SO WORTH IT! Please believe it. I might not know you, but I know God created your life & your soul for a purpose. Scream it out, you are worth it! // So sad to have learned about another life lost today. Remembering her vibrance & beauty. // If you're struggling in believing your purpose & worth, please talk to someone. We need you here. 1.800.272.TALK."
Fourth-season The Bachelorette winner Jesse Csincsak shared pictures of himself with McAllister on Facebook hours after her death was reported.
"It saddens me deeply to hear the news of my #friend Lex McAllister Taking her own life !! You were a beautiful person inside and out !!! You will be missed !! Until we meet again dear R.I.P gone but never forgotten !!!" Csincsak wrote.
If McAllister's death is definitely ruled a suicide, she'll be the second woman from Pavelka's season to take her own life. Third-place finisher Gia Allemand, who also later appeared on Bachelor Pad, hanged herself in August 2013.
The Bachelor airs every Monday night from 8-10PM ET/PT on ABC.