Low dose Doxycycline 20mg per day: Periostat: Is extremely safe. Rare side effect which I have never seen with doxycycline but has been reported at higher levels.

Low dose Doxycycline 20mg/d helps chronic dry eye (an inflammatory disease often) by inhibiting inflammatory factors (ie MMPs) with a mechanism which is independent from their antibiotic effect. However at high doses, there have been rare case reports of pseudotumor cerebri (intracranial hypertension).

How doxycycline causes intracranial hypertension is not known; however, case reports abound of increased intracranial pressure associated with drugs including tetracycline,7 minocycline8 and doxycycline.9 Proposed mechanisms by which “cyclines” (tetracycline, doxycycline, minocycline) cause intracranial hypertension include the drugs’ interference with the energy dependent absorption mechanism by affecting cyclic adenosine monophosphate at the arachnoid granulations.8 Although no clear genetic link exists to increased intracranial pressure, a genetic predisposition was proposed when dizygotic twin sisters developed the disorder. Both were taking tetracycline for acne. One was symptomatic with papilloedema, headache, and intracranial hypertension, while the other was found to have asymptomatic papilloedema after her twin had received her diagnosis.10 Most of what we know about benign intracranial hypertension concerns the idiopathic form. More common than previously recognised, idiopathic intracranial hypertension occurs in 10-20/100 000 obese women. This means that idiopathic intracranial hypertension is as common among obese women as multiple sclerosis. The disorder affects women (7:1), who present with symptoms of intracranial hypertension (headache, diplopia, whooshing noises in the head) and signs of intracranial hypertension (papilloedema, palsy of the sixth cranial nerve). Ninety per cent of the patients are obese.3
In contrast, intracranial hypertension due to the tetracycline antibiotics (including doxycycline) occurs in both sexes, at almost any age, and without concomitant obesity. The symptoms and signs of intracranial hypertension, however, are the same. How quickly a person develops intracranial hypertension after ingesting doxycycline is unknown, but in the largest review of intracranial hypertension induced by minocycline, some participants had used the drug for up to a year before developing 
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