Once an addict, always an addict. Or so the saying goes.
The Babadook is not a horror film... not to me, at least. It's a dark, brooding psychological drama that utilizes tropes of the horror genre to convey an underlying message. It's a film about two things: grief and drug addiction. This is quite clear to me. "You can't get rid of The Babadook." Certainly, you can't; you can't get rid of your addictions, and you can't get rid of the feeling of missing your deceased loved ones... you can only tame yourself to live without harmful, escapist tendencies. The ending of this film is in perfect concordance with this notion. Amelia is a woman haunted by the loss of her husband... that is the primary conflict here. But as a result of her loss, she became hooked on an easy escape - probably methamphetamine, from what I can tell. The tooth aches, the dilated eyes, the belief that substances can be effective problem solvers, the bi-polar behavior/ups and downs, the hallucinations... like I said, it's quite clear to me. The Babadook is a great film; it could be seen as a fine companion piece to another fantastic metaphorical film about methamphetamine addiction, William Friedkin's Bug. If you enjoyed this film, and have yet to see that one, I highly recommend checking it out. Jennifer Kent really went beyond the realm of horror with this one; rather than craft a film about a shadowy monster that haunts a mother and her son, she has created a brilliantly symbolic psychodrama that is both frightening and poignant, a commentary on grief/loss, chemical dependence and interpersonal relationships. Definitely a film that I look forward to revisiting.
Edit: if you're curious about how I arrived at my addiction theory, specific moments to pay attention to are (spoilers, obviously): 1) when she's entranced by the television/having hallucinations & delusions, 2) her inability to sleep, 3) the bugs crawling around her house/on her body while she's driving, 4) the way she switches from being super up to super down, 5) the way she seems so relieved when the doctor begins writing a prescription (more so than you'd think a mother dealing with a son who has emotional problems would be; the sigh she gives is almost like a "this will solve everything in the world" type of sigh), 6) the paranoia and facial twitches in the police station, 7) the pupil dilation scene (this is probably the strongest indication of drugs being involved; it occurs just before an hour and ten minutes into the film), 8) when she pulls her tooth out as if it had been decaying or something (there's also a foreshadowing to the teeth aspect when the boy grinds his teeth in bed toward the beginning), 9) toward the end of the movie when the boy has her tied up and she almost seems to be withdrawing ("you have to get it out!") and 10) the literal vomiting of the black ooze (black because she is removing the chemicals/toxins from her body) later on in this scene; it seems to me like this symbolizes purging something more than just her being absorbed in her grief - vomiting blood can be a side effect of methamphetamine addiction, and vomiting does occur during withdrawals, which is what, to me, seems to be occurring in this scene.