Is breaking the fast to take diarrhea medication during the daytime in Ramadan, given the presence of obsessive-compulsive disorder (OCD) which increases the hardship of fasting in this situation, considered a forbidden breaking of the fast, or is it excused, necessitating making up the days after Ramadan?

You must ignore the whispers (waswas) concerning purity. There is no sin in breaking the fast for one who experiences severe diarrhea and needs medication, if fasting would cause them an unbearable hardship. They must make up that day. It is permissible for a sick person to break their fast if fasting increases their illness, delays their recovery, or causes them hardship. Breaking the fast may even become obligatory if not doing so leads to death.