My recommendation is a disease resulting in the release of an acetylcholine antagonist into the blood stream. The source could be congenital or the result of a bacterial or viral infection. Such chemicals are produced by bacteria in nature and appear as components in the venom of many animals.
The action of the drug could either be to bind with, and neutralize the acetylcholine antagonist (like an anti-venom) or to attack and degrade the bacteria/virus. In the second case, much like HIV, the treatment is not sufficient to completely eliminate the pathogen from the body and stopping the treatment results in a rapid resurgence of the pathogen.
In either case, failure to take the required prophylactic treatment results in a spike in the amount of acetylcholine antagonist in the blood stream and the elimination of the neurotransmitter acetylcholine from the victims body. Dizziness and nausea, followed by loss of equilibrium and muscle control, followed by paralysis, followed by loss of respiration, heart function, and death.
Does this fit what you are looking for? I imagine you can use any clinical description of death by venom to provide the details for the crisis your character will undergo if denied treatment.
If this is too scifi, you could go with a viral or bacterial meningitis. With a slight mutation, you could get a viral meningitis that will be fatal if not kept in check with anti-viral drugs. Alternately, you could conceive of a bacterial infection that can be degraded but not eliminated with antibiotics due to the difficulty of treating the infection in the spinal column, some level of antibiotic resistance, or a combination of both.
With this infection, failure to continue treatment results in a fatal case of meningitis. The time required for the symptoms to become acute and then fatal could vary widely to fit your story. There are existing meningeal infections that cause coma and death in under 24 hours after infection, and there are those that require weeks to develop, with all points in between.