Glaucoma is among the leading causes for irreversible blindness worldwide. While lowering intraocular pressure (IOP) remains the mainstay of therapy, there are still some patients who progress despite wellcontrolled IOP. There is evidence from several studies that ocular blood flow and its regulation is impaired in patients with glaucoma. Tetrahydrocannabinol (THC) has been used in the treatment for glaucoma in some countries for several years due to its IOP lowering effect. In addition, there is also evidence that THC features neuroprotective effects and improves ocular hemodynamics. Dronabinol is a synthetic THC that is legally available in several European countries. It has the advantage that exact dosing of THC is possible in contrast to previously applied administration forms such as smoking. Due to its legal status in the past, data about the effect of THC on ocular blood flow and its regulation are sparse. This holds true for basic research in healthy subjects as well as in patients with glaucoma.The aim of the present study therefore is to investigate whether single administration of THC alters optic nerve head (ONH) blood flow in healthy subjects. In addition, other parameters for ocular blood flow will be measured, in particular retinal blood flow, retinal oxygen saturation and retinal neurovascular coupling. The study will be conducted in a randomized, double-masked, placebocontrolled, two-way cross-over design. Subjects will receive 5mg dronabinol on one study day. This dose is the recommended starting dose for some indications in clinical practice. Other studies investigating retinal hemodynamics or IOP after administration of THC also have used similar or slightly higher doses.

Inclusion Criteria:

• Men and women aged between 18 and 35 years
• Normal ophthalmic findings
• Ametropia ≤ 6 diopters
• Normal findings in the medical history and physical examination including ECG unless the investigator considers an abnormality to be clinically irrelevant
• Normal findings in the laboratory testing unless the investigator considers an abnormality to be clinically irrelevant
• Nonsmokers

Exclusion Criteria:

• Regular use of medication, abuse of alcoholic beverages or drugs
• History of drug or alcohol abuse
• Psychiatric disorders in the medical history
• Risk for drug dependence as evaluated by a psychiatrist
• Participation in a clinical trial in the 3 weeks preceding the study
• Positive urine drug test at the screening examination or on the study days
• Positive alcohol breath test at the screening examination or on the study days
• Regular consumption of cannabis and inability to not consume cannabis during the study period
• Treatment in the previous 3 weeks with any drug (except intake of oral contraceptives)
• Symptoms of a clinically relevant illness in the 3 weeks before the first study day
• History or presence of gastrointestinal, liver or kidney disease, or other conditions known to interfere with distribution, metabolism or excretion of study drugs
• Blood donation during the previous 3 weeks
• Known hypersensitivity to any of the components of the IMP under investigation or other study medication
• History or family history of epilepsy
• Pregnant or breast-feeding women
• Women of childbearing potential (neither menopausal, nor hysterectomized, nor sterilized) not using effective contraception (oral contraceptives, intra-uterine device, contraceptive implant or condoms)