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Usually for adults, the dosing is 800 mg. of sulfamethoxazole and 160 mg. of trimethoprim (SMZ/TMP) once a day for prevention of PCP (Pneumocystis carinii pneumonia.)
In answer to your first question...yes, a doctor would be acting appropriately in prescribing smz/tmp as a preventative for PCP three times a week. It is with infants and children that the preventative dose for PCP is based on body size (which is determined by the doctor.) There are several dosing regimens. One regimen is what you mentioned in your post, and consists of 375 mg. of sulfamethoxazole and 75 mg. of trimethoprim per square meter of body surface twice a day, three times a week for three consecutive days in a row.
SMZ/TMP has other uses such as for urinary infections, middle ear infections, prostatitis, pneumonitis, and bronchitis.
Pneumonitis and bronchitis are other types of lung infections. Both can be chronic, however it is not likely to be prescribed SMZ/TMP for preventative use for these infections as the drug can be toxic to the kidneys and risks of blood disorders.
Please write back if you need further clarification.