A woman went viral for using a tampon's serial number to find out whether her boyfriend cheated, dividing opinion among millions of viewers

• Lois Saunders posted a TikTok video explaining how she tried to learn whether her boyfriend cheated.
• After finding a tampon under his closet, she contacted the manufacturer with the serial number.

Internet relationship sleuthing reached new heights when a 23-year-old TikTok user named Lois Saunders uploaded a video on Monday explaining how she tracked down a tampon's serial number to try to find out whether her boyfriend had cheated on her.

The video, which has been viewed 4.4 million times and has more than 450,000 likes, shows Saunders crawling on the floor under a closet, while on-screen text says, "When you find a tampon and mascara under you bf wardrobe and they deny everything, you have to investigate for yourself."

@loissa4 #tampax ♬ original sound - Lois Sa

Saunders, who lives in the English county of Essex, also showed screenshots of email correspondence between her and a representative for Procter & Gamble, the parent company for Tampax. "I have a tampon with the code 93452080N2 on the side. It's yellow; regular. Can this tell me when it was manufactured?" her original email said.

She then showed a lengthy response from a member of the customer-support team who informed Saunders that the tampon was manufactured on December 11, 2019. A representative for Procter & Gamble confirmed the authenticity of the emails in a statement to Insider.

If the tampon was manufactured after they began dating, she thought it would have been "a funny way to find out he was unfaithful," she told Insider, adding that technically they weren't a couple at the time.

Saunders' sleuthing skills quickly went viral, with Tampax's official account referring to her in a comment under the video as an "international super spy." Other most liked comments called the story "amazing" and said they were "so invested" in finding out what happened next.

Later the same day, Saunders uploaded two more TikToks explaining the situation, saying that several girls had previously lived in the house and that she believed that was the explanation behind the tampon and mascara.

She also revealed that she and her boyfriend, who requested we refer to him by only his first name, Fin, because of privacy concerns, were not in a relationship when the events took place in May 2021, as the couple were "having a break" while at college in different parts of the country, she told Insider.

This led some viewers to leave comments suggesting she'd gone "too far."

Fin also said the response had been "mixed." In a statement to Insider, he said some viewers tracked down his social media and sent messages "warning" him about Saunders and telling him to pack his bags "and run." Others believed he had cheated and were calling for him to be "canceled."

Still, Fin says he has "no regrets" regarding the video and the response messages. "Some of them I found absolutely hilarious," Fin told Insider. "Complete strangers would be arguing in the comments, fighting my corner, and making wild and crazy stories that are so far from the truth."

Saunders told Insider she contacted Tampax "purely as a joke" and had "full trust" in Fin, who said he made the video because "seeing her on all fours looking under that huge wardrobe was hilarious."

"Cheating boyfriend" videos are a widely popular genre on TikTok. Uploads under the #caughtcheating tag have 1.3 billion views on the app, with #cheatingboyfriend at 968.4 million. In September, a video of a woman surprising her boyfriend — later dubbed "Couch Guy" — went hugely viral as viewers debated whether he was cheating on her based on supposed clues in the clip. The video has over 65 million views and was even discussed on "The View."

Despite this, Saunders said she didn't expect to go viral, telling Insider: "Straight away after minutes it started getting way more attention than other TikToks I had put up previously. Then when my friends started saying they had seen it is when I realized how big it was."

Fin told Insider he came across the video only when looking through his Snapchat stories from the year. "There was absolutely no intention of ever 'planning' this for TikTok," he said. "No way could I have seen it blowing up."

For more stories like this, check out coverage from Insider's Digital Culture team here.