Half-wolf, half-man. That's what the Werelox virus made you, no matter your species. The Doctor was slowly turning into a Werelok. He couldn't stay with Sharon on New Earth. He left with K-9 in the TARDIS, telling him to zap him if he went out of control.
For three months, the Doctor worked on a cure to save himself. It was hard. The Werelox virus threatened to take over his brain and cause him to stop thinking (somewhat) rationally. Whenever he'd throw around items in the TARDIS' lab, K-9 zapped him. The Doctor's relapses as well as K-9 stunning him would delay his progress.
The Doctor did finish his antidote. He drunk it and was stunned, without K-9's help, for a final time.
After waking up, the Doctor was a Werelok no more. He returned to face the Werelox and to find Sharon.
Much to his surprise, he returned ten minutes after he left Sharon.
The Doctor shrugged it off and returned to the fray, happy that, for once, he was able to manoeuvre the TARDIS to where it needed to be—and early to boot!