It certainly can be, just as many other drugs such as alcohol, caffeine, and cocaine, especially if used in inappropriate ways. Below are some of the wikipedia listed effects...

Anti-emetic properties Modulation of working and short-term memory Impairment of short-term memory Enhancement of many other drug effects (including those of alcohol, ecstasy, tobacco, heroin, cocaine) Auditory or visual hallucinations at high doses in some users Paramnesia, repetitiveness and ambiguation Increased appetite (often referred to as "the munchies"), an effect of stimulation of the endocannabinoid system, which affects body weight, insulin resistance, and dyslipidemia. Increased sexual pleasure Increased awareness of sensation Increased awareness of patterns and colors Increased appreciation of music Increased mental activity, like metacognition Induced sense of novelty Initial wakefulness followed by drowsiness and lassitude ('burnt out') Introspective or meditative states of mind Gain or loss of some inhibitions Mild euphoria, feelings of general well-being Relaxation or stress reduction Mild tachycardia (Increased heart rate) Mild, temporary dry mouth (sometimes referred to as cottonmouth, pasties, or the drys (NZ)) Mild, temporary keratoconjunctivitis sicca (sometimes referred to as blood-shot eyes, dry eyes or red eye(UK)) Mild paranoia and anxiety
Most of all: it is best for the world??s number-one killer - stress. It can safely curtail or replace Valium, Librium, alcohol, or even Prozac, for millions of Americans.

While cannabis intoxication varies with psychological set and social setting, ??the most common response is a calm, mildly euphoric state in which time slows and a sensitivity to sights, sounds and touch is enhanced.?

In contrast to marijuana??s safe, therapeutic action, benzodiazephines (Valium) abuse is the number-one drug abuse problem in the country, and is responsible for more emergency room admissions in the United States than either cocaine-related problems or morphine and heroin-related admissions combined.

While tobacco constricts arteries, cannabis dilates (opens) them. Because migraine headaches are the result of artery spasms combined with over-relaxation of veins, the vascular changes cannabis causes in the covering of the brain (the meninges) usually made migraines disappear.

Evidence of vascular change caused by cannabis can be seen in the user??s red eyes, which are extensions of the brain. However, unlike most other drugs, cannabis has no apparent effect on the vascular system in general, except for a slight increased heart rate during the onset of the high.
No website i could find actually gives a proper scientific explanation why, interesting because i dont know the actual reason myself but i would assume its simple because weed, THC trichs have chemicals in them which dehydrate you and make you have less saliva and less water around your eyes and mouth and whole body in general.