It's hard for any new hepatitis C drugs to compete with Gilead Sciences' (NASDAQ:GILD) Harvoni, with its cure rate topping 95%, especially when you consider that some of those uncured patients are the inevitable dropouts who can be found in every clinical trial, no matter how good the drug is.

Drugs with equal efficacy can still compete if they have a better side effect profile than the standard of care, but Harvoni is pretty darn safe. The most common adverse reactions in the drug's clinical trials were fatigue and headache. The only warning on the label is to not combine it with Sovaldi, one of the ingredients of the Harvoni cocktail, to avoid double dosing.

From a clinical perspective, the only thing left to compete on is convenience. Interrupting treatment before a cure is established not only costs insurance companies money but can also select for viruses that are resistant to the medication. All things equal, doctors would prefer to prescribe the drug that is the easiest for patients to take.

Source: Gilead Sciences

Harvoni is dosed once daily for eight or 12 weeks for most patients. If a drug can cut treatment to four or six weeks, there should be better compliance. Fortunately for Gilead Sciences, Merck's (NYSE:MRK) newest combination doesn't seem to be working particularly well for the shorter durations.

This week at the American Association for the Study of Liver Diseases, or AASLD, annual meeting, the company released data from a clinical trial testing Merck's grazoprevir and elbasvir combined with Gilead Sciences' Sovaldi.

The triple combination cured just 38.7% of patients who were treated for four weeks. That's not going to cut it.

Over a six-week treatment regiment, the combo cured 86.7% of patients who had no cirrhosis of the liver and 80% of patients who were cirrhotic. That still doesn't compare too favorably with the 94% cure rate in Harvoni's eight-week clinical trial, but keep in mind that this is an early trial with just 20-30 patients in each subgroup. A couple of cures one way or another can make a big difference on the cure rates.

It sounds like Merck might be planning a larger trial to test six weeks of treatment, as well as eight weeks, which initially produced a cure rate of 94.7% in cirrhotic patients (when the two patients who discontinued use are subtracted).

It's probably worth testing the combination for six weeks in case Merck got statistically unlucky in the smaller trial, but if the cure rate comes in under 90%, then it would unlikely for doctors to prescribe six weeks' worth of Merck's combination over eight weeks of Harvoni.

Gilead isn't out of the woods quite yet. Merck has a drug called MK-8408, in the same class as elbasvir, which it believes could be more potent. There's still potential for Merck to break the eight-week barrier with cure rates above 90%, although the combination would take longer to get on the market than elbasvir-containing combinations.

Gilead could also see competition from Bristol-Myers Squibb (NYSE:BMY), which is testing its hepatitis C drugs in combination with Sovaldi for four weeks. We should get data from that trial next year.

Gilead Sciences isn't resting on its laurels. A clinical trial testing Gilead's GS-9669 and GS-9451 with Harvoni showed that the cocktails could produce cure rates of 95%-100% with just six weeks of treatment. The company is also testing Harvoni-containing cocktails for as little as four weeks.

By this time next year, perhaps even a little earlier, we should have more insight into the front runner for short-duration treatment. For now, Gilead Sciences and its investors can breathe easy.