May one take aspirin soluble in water on shabbos, if feeling cold?

There is no difference between soluble medicines and pills. Most authorities prohibit it unless one either must go to sleep to avoid the pain or his whole body aches- in these cases all allow. Some authorities (R. Yonatan Steif) allow it even for minor pain, such as just feeling cold so if it's really necessary, one can rely on the more lenient rabbis.