Is expiation obligatory for one who broke their fast in Ramadan due to forgetfulness and not taking medication, then had intercourse with their wife who was also breaking her fast for a legitimate religious reason on the same day, or is only making up the missed day sufficient?

If you broke your fast due to a valid excuse that permits it, which is your fear of harm from not taking medication, then once that excuse is gone, scholars differ on whether you are required to abstain [from eating and drinking for the rest of the day] or not. Those who say that abstention is obligatory in this case would require you to pay expiation (kaffarah) if you engaged in intercourse. As for those who say that abstention is not obligatory, you would not owe expiation; you would only need to make up the missed fasts (qada'). The most preponderant opinion among scholars is that abstention is not obligatory in this situation. This is also the view favored by the esteemed scholar Al-Uthaymeen. Consequently, you do not owe expiation, and you are only required to make up the missed fasts.