Section 724.011 of the Texas State Transportation Code states that anyone who is arrested for Texas DWI "is deemed to have consented, subject to this chapter, to submit to the taking of one or more specimens of the person's breath or blood for analysis to determine the alcohol concentration or the presence in the person's body of a controlled substance, drug, dangerous drug, or other substance." Basically, whenever you stick your keys in your ignition and start your car, you are consenting to take an alcohol or drug test if a police officer deems it necessary.
What happens if you refuse? The police cannot make you take the test by force unless you have caused an accident that left someone else seriously injured or dead, but you will be found guilty of violating the "implied consent" section of the Transportation Code. You can still be charged for DWI even if you don't submit to a breath or blood test, and the state will try to prove your guilt through other means. However, in addition to potential Texas DWI conviction, you will also face the following penalties for refusing a chemical test: The officer will take your driver's license and issue you a temporary driving permit. On the 41st day after the arrest, your license will be suspended for 180 days unless you request a hearing first. If you have previously been arrested for DWI and refused to provide a specimen, the length of the administrative suspension jumps to 2 years.