Preventive Medicine on Shabbos
Is one allowed to take medicine on shabbos if one is healthy and is just taking medicine to prevent a future ailment (i.e. taking medicine for allergies before the allergy symptoms start)?
Someone who has an oncoming ailment and takes medicine to prevent it, would be included in the opinion of the Magen Avraham that taking medicine “לחזק מזגו” to strengthen himself is prohibited. While some Achronim argue on this Magen Avraham [see Igros Moshe 2-54], the Mishna Brura does quote his opinion. See Shulchan Aruch 328:34.
If the oncoming ailment is one which will make him bedridden or completely sick, he would be permitted in advance to take the medicine to prevent this illness.
In addition one who takes a medicine regularly, such as allergy medicine could be lenient to take preventive medicine.