Britney Spears had pills in her possession when she was arrested for DUI … and TMZ has learned those pills are in the process of being tested, and the results could theoretically mean jail time.

Sources familiar with the situation tell TMZ … the pills were Adderall — something she got while recently in Mexico. Our sources say one of the reasons Britney goes down to Mexico so often is to obtain the drug.

Our sources tell us … those pills will be tested to determine if they contain pure Adderall or an additional substance. It’s not uncommon that certain drugs obtained in Mexico are laced with fentanyl, cocaine, meth or other substances.

Although CHP told us they did not find substances in Britney’s car during the arrest, our sources say the pills were indeed found and are now in the process of being tested.

This is Britney’s first DUI, and typically judges will not order jail time. However, if the drugs in her system contain serious levels of narcotics, judges have discretion to order that the offender spend time behind bars.

Britney’s manager Cade Hudson tells TMZ, “This was an unfortunate incident that is completely inexcusable. Britney is going to take the right steps and comply with the law and hopefully this can be the first step in long overdue change that needs to occur in Britney’s life. Hopefully, she can get the help and support she needs during this difficult time.”