According to the United Kingdom's NHS, it is safe to drink alcohol when taking trimethoprim. The NHS explains that alcohol does not interfere with the function of this specific antibiotic.
The NHS indicates that some drugs do interact negatively with alcohol. The Mayo Clinic warns that when trimethoprim is combined with sulfamethoxazole, as in the prescription drug Bactrim, alcohol should not be used. Patients sometimes suffer moderate to severe effects, including flushing, headache, nausea, vomiting and elevated heart rate. The alert from the Mayo Clinic does not include just alcoholic beverages. Alcohol is an ingredient is some over-the-counter medications and mouth washes, as well. The Mayo Clinic warns that alcohol of any kind should be avoided when taking trimethoprim combined with sulfamethoxazole.