Virithipa "VJ Woonsen" Pakdeeprasong and Nonthaporn "Ying Yae" Theerawattanasook admitted Sunday they voluntarily posted pictures of themselves with bottles of Chang Beer to promote the product on their Instagram accounts.
The pair, however, denied being paid by the brewing firm, claiming they only wanted to help a close friend promote the product.
Police did not disclose their friend's name.
Ms Virithipa and Ms Nonthaporn appeared at the Office of the Consumer Protection Board (OCPB) to testify with deputy police chief Pol Gen Pongsapat Pongcharoen about the alleged social media campaign.
The probe was launched after the pair and 22 other celebrities were found to have posted photos of themselves holding Chang Beer during the same period on Instagram. All carried similar messages encouraging the public to drink the beer. The incident has raised suspicions of tacit advertising aimed at social media users.
While the brewery denies encouraging the celebrities to push its products, critics see the incident as a subtle social media campaign, using celebrities posting images of themselves with bottles of the beer.
Singer and actor Pakorn "Dome" Lam was among the 24 celebrities.
Seven of them, including Waranchaya "VJ Ja" Chindarakwong and Virithipa, also appeared on Chang's website to promote the beer's new bottle colour.
Pol Gen Pongsapat said police have not yet pressed charges against the 24 celebrities.
The deputy police chief said he will speak on Monday to the Department of Disease Control and the Food and Drug Administration to see which sections of the law they may have violated.
If they acted intentionally for a commercial purpose, legal action may be taken against them.
Meanwhile, Sarawut Benjakul, head of the Institute of Legal Education of the Thai Bar Association, said the intention behind the Instagram posts would be key to deciding whether they violated the law.
If they did not intend to encourage the public to drink, they will not face punishment, said Mr Sarawut.
The Alcohol Control Act 2008 law bans anyone from advertising alcoholic drinks and from showing the names and logos of alcoholic drinks to promote them.
Producers of alcoholic drinks are only allowed to advertise products by giving useful information to society without showing the products' pictures, packaging or containers, the law says.
Those who break the law will face a jail term of up to one year and/or a fine of up to 500,000 baht, or a daily fine of 50,000 baht throughout the period of the violation.
Mr Sarawut said legal action will be brought against offenders who share photos of alcoholic drinks with the intention of enticing the public to consume alcohol both directly and indirectly, as the Criminal Code also applies.
People sharing with their friends photos of alcoholic drinks with barely visible logos will not be charged if they aren't trying to persuade others to drink, Mr Sarawut said.