Subtherapeutic means not enough to treat a sickness.
Subtherapeutic can make people think of not being strong enough to help.
Examples:
The doctor said the medicine was subtherapeutic.
She took subtherapeutic doses of the medicine.
The subtherapeutic treatment didn't help at all.
The subtherapeutic soup didn't make me feel better.
The subtherapeutic bandage didn't stop the bleeding.
The subtherapeutic medicine was like drinking water.